Active Record Associations

Associations are a set of macro-like class methods for tying objects together through foreign keys. They express relationships like “Project has one Project Manager” or “Project belongs to a Portfolio”. Each macro adds a number of methods to the class which are specialized according to the collection or association symbol and the options hash. It works much the same way as Ruby’s own attr* methods.

class Project < ActiveRecord::Base
  belongs_to              :portfolio
  has_one                 :project_manager
  has_many                :milestones
  has_and_belongs_to_many :categories
end

The project class now has the following methods (and more) to ease the traversal and manipulation of its relationships:

project = Project.first
project.portfolio
project.portfolio = Portfolio.first
project.reload_portfolio

project.project_manager
project.project_manager = ProjectManager.first
project.reload_project_manager

project.milestones.empty?
project.milestones.size
project.milestones
project.milestones << Milestone.first
project.milestones.delete(Milestone.first)
project.milestones.destroy(Milestone.first)
project.milestones.find(Milestone.first.id)
project.milestones.build
project.milestones.create

project.categories.empty?
project.categories.size
project.categories
project.categories << Category.first
project.categories.delete(category1)
project.categories.destroy(category1)

A word of warning

Don’t create associations that have the same name as instance methods of ActiveRecord::Base. Since the association adds a method with that name to its model, using an association with the same name as one provided by ActiveRecord::Base will override the method inherited through ActiveRecord::Base and will break things. For instance, attributes and connection would be bad choices for association names, because those names already exist in the list of ActiveRecord::Base instance methods.

Auto-generated methods

See also “Instance Public methods” below ( from belongs_to ) for more details.

Singular associations (one-to-one)

                                  |            |  belongs_to  |
generated methods                 | belongs_to | :polymorphic | has_one
----------------------------------+------------+--------------+---------
other                             |     X      |      X       |    X
other=(other)                     |     X      |      X       |    X
build_other(attributes={})        |     X      |              |    X
create_other(attributes={})       |     X      |              |    X
create_other!(attributes={})      |     X      |              |    X
reload_other                      |     X      |      X       |    X
other_changed?                    |     X      |      X       |
other_previously_changed?         |     X      |      X       |

Collection associations (one-to-many / many-to-many)

                                  |       |          | has_many
generated methods                 | habtm | has_many | :through
----------------------------------+-------+----------+----------
others                            |   X   |    X     |    X
others=(other,other,...)          |   X   |    X     |    X
other_ids                         |   X   |    X     |    X
other_ids=(id,id,...)             |   X   |    X     |    X
others<<                          |   X   |    X     |    X
others.push                       |   X   |    X     |    X
others.concat                     |   X   |    X     |    X
others.build(attributes={})       |   X   |    X     |    X
others.create(attributes={})      |   X   |    X     |    X
others.create!(attributes={})     |   X   |    X     |    X
others.size                       |   X   |    X     |    X
others.length                     |   X   |    X     |    X
others.count                      |   X   |    X     |    X
others.sum(*args)                 |   X   |    X     |    X
others.empty?                     |   X   |    X     |    X
others.clear                      |   X   |    X     |    X
others.delete(other,other,...)    |   X   |    X     |    X
others.delete_all                 |   X   |    X     |    X
others.destroy(other,other,...)   |   X   |    X     |    X
others.destroy_all                |   X   |    X     |    X
others.find(*args)                |   X   |    X     |    X
others.exists?                    |   X   |    X     |    X
others.distinct                   |   X   |    X     |    X
others.reset                      |   X   |    X     |    X
others.reload                     |   X   |    X     |    X

Overriding generated methods

Association methods are generated in a module included into the model class, making overrides easy. The original generated method can thus be called with super:

class Car < ActiveRecord::Base
  belongs_to :owner
  belongs_to :old_owner

  def owner=(new_owner)
    self.old_owner = self.owner
    super
  end
end

The association methods module is included immediately after the generated attributes methods module, meaning an association will override the methods for an attribute with the same name.

Cardinality and associations

Active Record associations can be used to describe one-to-one, one-to-many, and many-to-many relationships between models. Each model uses an association to describe its role in the relation. The belongs_to association is always used in the model that has the foreign key.

One-to-one

Use has_one in the base, and belongs_to in the associated model.

class Employee < ActiveRecord::Base
  has_one :office
end
class Office < ActiveRecord::Base
  belongs_to :employee    # foreign key - employee_id
end

One-to-many

Use has_many in the base, and belongs_to in the associated model.

class Manager < ActiveRecord::Base
  has_many :employees
end
class Employee < ActiveRecord::Base
  belongs_to :manager     # foreign key - manager_id
end

Many-to-many

There are two ways to build a many-to-many relationship.

The first way uses a has_many association with the :through option and a join model, so there are two stages of associations.

class Assignment < ActiveRecord::Base
  belongs_to :programmer  # foreign key - programmer_id
  belongs_to :project     # foreign key - project_id
end
class Programmer < ActiveRecord::Base
  has_many :assignments
  has_many :projects, through: :assignments
end
class Project < ActiveRecord::Base
  has_many :assignments
  has_many :programmers, through: :assignments
end

For the second way, use has_and_belongs_to_many in both models. This requires a join table that has no corresponding model or primary key.

class Programmer < ActiveRecord::Base
  has_and_belongs_to_many :projects       # foreign keys in the join table
end
class Project < ActiveRecord::Base
  has_and_belongs_to_many :programmers    # foreign keys in the join table
end

Choosing which way to build a many-to-many relationship is not always simple. If you need to work with the relationship model as its own entity, use has_many :through. Use has_and_belongs_to_many when working with legacy schemas or when you never work directly with the relationship itself.

Is it a belongs_to or has_one association?

Both express a 1-1 relationship. The difference is mostly where to place the foreign key, which goes on the table for the class declaring the belongs_to relationship.

class User < ActiveRecord::Base
  # I reference an account.
  belongs_to :account
end

class Account < ActiveRecord::Base
  # One user references me.
  has_one :user
end

The tables for these classes could look something like:

CREATE TABLE users (
  id bigint NOT NULL auto_increment,
  account_id bigint default NULL,
  name varchar default NULL,
  PRIMARY KEY  (id)
)

CREATE TABLE accounts (
  id bigint NOT NULL auto_increment,
  name varchar default NULL,
  PRIMARY KEY  (id)
)

Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.

You can set the :autosave option on a has_one, belongs_to, has_many, or has_and_belongs_to_many association. Setting it to true will always save the members, whereas setting it to false will never save the members. More details about :autosave option is available at AutosaveAssociation.

One-to-one associations

  • Assigning an object to a has_one association automatically saves that object and the object being replaced (if there is one), in order to update their foreign keys - except if the parent object is unsaved (new_record? == true).

  • If either of these saves fail (due to one of the objects being invalid), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.

  • If you wish to assign an object to a has_one association without saving it, use the #build_association method (documented below). The object being replaced will still be saved to update its foreign key.

  • Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.

Collections

  • Adding an object to a collection (has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.

  • If saving any of the objects being added to a collection (via push or similar) fails, then push returns false.

  • If saving fails while replacing the collection (via association=), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.

  • You can add an object to a collection without automatically saving it by using the collection.build method (documented below).

  • All unsaved (new_record? == true) members of the collection are automatically saved when the parent is saved.

Customizing the query

Associations are built from Relation objects, and you can use the Relation syntax to customize them. For example, to add a condition:

class Blog < ActiveRecord::Base
  has_many :published_posts, -> { where(published: true) }, class_name: 'Post'
end

Inside the -> { ... } block you can use all of the usual Relation methods.

Accessing the owner object

Sometimes it is useful to have access to the owner object when building the query. The owner is passed as a parameter to the block. For example, the following association would find all events that occur on the user’s birthday:

class User < ActiveRecord::Base
  has_many :birthday_events, ->(user) { where(starts_on: user.birthday) }, class_name: 'Event'
end

Note: Joining or eager loading such associations is not possible because those operations happen before instance creation. Such associations can be preloaded, but doing so will perform N+1 queries because there will be a different scope for each record (similar to preloading polymorphic scopes).

Association callbacks

Similar to the normal callbacks that hook into the life cycle of an Active Record object, you can also define callbacks that get triggered when you add an object to or remove an object from an association collection.

class Firm < ActiveRecord::Base
  has_many :clients,
           dependent: :destroy,
           after_add: :congratulate_client,
           after_remove: :log_after_remove

  def congratulate_client(record)
    # ...
  end

  def log_after_remove(record)
    # ...
  end
end

It’s possible to stack callbacks by passing them as an array. Example:

class Firm < ActiveRecord::Base
  has_many :clients,
           dependent: :destroy,
           after_add: [:congratulate_client, -> (firm, record) { firm.log << "after_adding#{record.id}" }],
           after_remove: :log_after_remove
end

Possible callbacks are: before_add, after_add, before_remove, and after_remove.

If any of the before_add callbacks throw an exception, the object will not be added to the collection.

Similarly, if any of the before_remove callbacks throw an exception, the object will not be removed from the collection.

Note: To trigger remove callbacks, you must use destroy / destroy_all methods. For example:

  • firm.clients.destroy(client)

  • firm.clients.destroy(*clients)

  • firm.clients.destroy_all

delete / delete_all methods like the following do not trigger remove callbacks:

  • firm.clients.delete(client)

  • firm.clients.delete(*clients)

  • firm.clients.delete_all

Association extensions

The proxy objects that control the access to associations can be extended through anonymous modules. This is especially beneficial for adding new finders, creators, and other factory-type methods that are only used as part of this association.

class Account < ActiveRecord::Base
  has_many :people do
    def find_or_create_by_name(name)
      first_name, last_name = name.split(" ", 2)
      find_or_create_by(first_name: first_name, last_name: last_name)
    end
  end
end

person = Account.first.people.find_or_create_by_name("David Heinemeier Hansson")
person.first_name # => "David"
person.last_name  # => "Heinemeier Hansson"

If you need to share the same extensions between many associations, you can use a named extension module.

module FindOrCreateByNameExtension
  def find_or_create_by_name(name)
    first_name, last_name = name.split(" ", 2)
    find_or_create_by(first_name: first_name, last_name: last_name)
  end
end

class Account < ActiveRecord::Base
  has_many :people, -> { extending FindOrCreateByNameExtension }
end

class Company < ActiveRecord::Base
  has_many :people, -> { extending FindOrCreateByNameExtension }
end

Some extensions can only be made to work with knowledge of the association’s internals. Extensions can access relevant state using the following methods (where items is the name of the association):

  • record.association(:items).owner - Returns the object the association is part of.

  • record.association(:items).reflection - Returns the reflection object that describes the association.

  • record.association(:items).target - Returns the associated object for belongs_to and has_one, or the collection of associated objects for has_many and has_and_belongs_to_many.

However, inside the actual extension code, you will not have access to the record as above. In this case, you can access proxy_association. For example, record.association(:items) and record.items.proxy_association will return the same object, allowing you to make calls like proxy_association.owner inside association extensions.

Association Join Models

Has Many associations can be configured with the :through option to use an explicit join model to retrieve the data. This operates similarly to a has_and_belongs_to_many association. The advantage is that you’re able to add validations, callbacks, and extra attributes on the join model. Consider the following schema:

class Author < ActiveRecord::Base
  has_many :authorships
  has_many :books, through: :authorships
end

class Authorship < ActiveRecord::Base
  belongs_to :author
  belongs_to :book
end

@author = Author.first
@author.authorships.collect { |a| a.book } # selects all books that the author's authorships belong to
@author.books                              # selects all books by using the Authorship join model

You can also go through a has_many association on the join model:

class Firm < ActiveRecord::Base
  has_many   :clients
  has_many   :invoices, through: :clients
end

class Client < ActiveRecord::Base
  belongs_to :firm
  has_many   :invoices
end

class Invoice < ActiveRecord::Base
  belongs_to :client
end

@firm = Firm.first
@firm.clients.flat_map { |c| c.invoices } # select all invoices for all clients of the firm
@firm.invoices                            # selects all invoices by going through the Client join model

Similarly you can go through a has_one association on the join model:

class Group < ActiveRecord::Base
  has_many   :users
  has_many   :avatars, through: :users
end

class User < ActiveRecord::Base
  belongs_to :group
  has_one    :avatar
end

class Avatar < ActiveRecord::Base
  belongs_to :user
end

@group = Group.first
@group.users.collect { |u| u.avatar }.compact # select all avatars for all users in the group
@group.avatars                                # selects all avatars by going through the User join model.

An important caveat with going through has_one or has_many associations on the join model is that these associations are read-only. For example, the following would not work following the previous example:

@group.avatars << Avatar.new   # this would work if User belonged_to Avatar rather than the other way around
@group.avatars.delete(@group.avatars.last)  # so would this

Setting Inverses

If you are using a belongs_to on the join model, it is a good idea to set the :inverse_of option on the belongs_to, which will mean that the following example works correctly (where tags is a has_many :through association):

@post = Post.first
@tag = @post.tags.build name: "ruby"
@tag.save

The last line ought to save the through record (a Tagging). This will only work if the :inverse_of is set:

class Tagging < ActiveRecord::Base
  belongs_to :post
  belongs_to :tag, inverse_of: :taggings
end

If you do not set the :inverse_of record, the association will do its best to match itself up with the correct inverse. Automatic inverse detection only works on has_many, has_one, and belongs_to associations.

:foreign_key and :through options on the associations will also prevent the association’s inverse from being found automatically, as will a custom scopes in some cases. See further details in the Active Record Associations guide.

The automatic guessing of the inverse association uses a heuristic based on the name of the class, so it may not work for all associations, especially the ones with non-standard names.

You can turn off the automatic detection of inverse associations by setting the :inverse_of option to false like so:

class Tagging < ActiveRecord::Base
  belongs_to :tag, inverse_of: false
end

Nested Associations

You can actually specify any association with the :through option, including an association which has a :through option itself. For example:

class Author < ActiveRecord::Base
  has_many :posts
  has_many :comments, through: :posts
  has_many :commenters, through: :comments
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :commenter
end

@author = Author.first
@author.commenters # => People who commented on posts written by the author

An equivalent way of setting up this association this would be:

class Author < ActiveRecord::Base
  has_many :posts
  has_many :commenters, through: :posts
end

class Post < ActiveRecord::Base
  has_many :comments
  has_many :commenters, through: :comments
end

class Comment < ActiveRecord::Base
  belongs_to :commenter
end

When using a nested association, you will not be able to modify the association because there is not enough information to know what modification to make. For example, if you tried to add a Commenter in the example above, there would be no way to tell how to set up the intermediate Post and Comment objects.

Polymorphic Associations

Polymorphic associations on models are not restricted on what types of models they can be associated with. Rather, they specify an interface that a has_many association must adhere to.

class Asset < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true
end

class Post < ActiveRecord::Base
  has_many :assets, as: :attachable         # The :as option specifies the polymorphic interface to use.
end

@asset.attachable = @post

This works by using a type column in addition to a foreign key to specify the associated record. In the Asset example, you’d need an attachable_id integer column and an attachable_type string column.

Using polymorphic associations in combination with single table inheritance (STI) is a little tricky. In order for the associations to work as expected, ensure that you store the base model for the STI models in the type column of the polymorphic association. To continue with the asset example above, suppose there are guest posts and member posts that use the posts table for STI. In this case, there must be a type column in the posts table.

Note: The attachable_type= method is being called when assigning an attachable. The class_name of the attachable is passed as a String.

class Asset < ActiveRecord::Base
  belongs_to :attachable, polymorphic: true

  def attachable_type=(class_name)
     super(class_name.constantize.base_class.to_s)
  end
end

class Post < ActiveRecord::Base
  # because we store "Post" in attachable_type now dependent: :destroy will work
  has_many :assets, as: :attachable, dependent: :destroy
end

class GuestPost < Post
end

class MemberPost < Post
end

Caching

All of the methods are built on a simple caching principle that will keep the result of the last query around unless specifically instructed not to. The cache is even shared across methods to make it even cheaper to use the macro-added methods without worrying too much about performance at the first go.

project.milestones             # fetches milestones from the database
project.milestones.size        # uses the milestone cache
project.milestones.empty?      # uses the milestone cache
project.milestones.reload.size # fetches milestones from the database
project.milestones             # uses the milestone cache

Eager loading of associations

Eager loading is a way to find objects of a certain class and a number of named associations. It is one of the easiest ways to prevent the dreaded N+1 problem in which fetching 100 posts that each need to display their author triggers 101 database queries. Through the use of eager loading, the number of queries will be reduced from 101 to 2.

class Post < ActiveRecord::Base
  belongs_to :author
  has_many   :comments
end

Consider the following loop using the class above:

Post.all.each do |post|
  puts "Post:            " + post.title
  puts "Written by:      " + post.author.name
  puts "Last comment on: " + post.comments.first.created_on
end

To iterate over these one hundred posts, we’ll generate 201 database queries. Let’s first just optimize it for retrieving the author:

Post.includes(:author).each do |post|

This references the name of the belongs_to association that also used the :author symbol. After loading the posts, find will collect the author_id from each one and load all of the referenced authors with one query. Doing so will cut down the number of queries from 201 to 102.

We can improve upon the situation further by referencing both associations in the finder with:

Post.includes(:author, :comments).each do |post|

This will load all comments with a single query. This reduces the total number of queries to 3. In general, the number of queries will be 1 plus the number of associations named (except if some of the associations are polymorphic belongs_to - see below).

To include a deep hierarchy of associations, use a hash:

Post.includes(:author, { comments: { author: :gravatar } }).each do |post|

The above code will load all the comments and all of their associated authors and gravatars. You can mix and match any combination of symbols, arrays, and hashes to retrieve the associations you want to load.

All of this power shouldn’t fool you into thinking that you can pull out huge amounts of data with no performance penalty just because you’ve reduced the number of queries. The database still needs to send all the data to Active Record and it still needs to be processed. So it’s no catch-all for performance problems, but it’s a great way to cut down on the number of queries in a situation as the one described above.

Since only one table is loaded at a time, conditions or orders cannot reference tables other than the main one. If this is the case, Active Record falls back to the previously used LEFT OUTER JOIN based strategy. For example:

Post.includes([:author, :comments]).where(['comments.approved = ?', true])

This will result in a single SQL query with joins along the lines of: LEFT OUTER JOIN comments ON comments.post_id = posts.id and LEFT OUTER JOIN authors ON authors.id = posts.author_id. Note that using conditions like this can have unintended consequences. In the above example, posts with no approved comments are not returned at all because the conditions apply to the SQL statement as a whole and not just to the association.

You must disambiguate column references for this fallback to happen, for example order: "author.name DESC" will work but order: "name DESC" will not.

If you want to load all posts (including posts with no approved comments), then write your own LEFT OUTER JOIN query using ON:

Post.joins("LEFT OUTER JOIN comments ON comments.post_id = posts.id AND comments.approved = '1'")

In this case, it is usually more natural to include an association which has conditions defined on it:

class Post < ActiveRecord::Base
  has_many :approved_comments, -> { where(approved: true) }, class_name: 'Comment'
end

Post.includes(:approved_comments)

This will load posts and eager load the approved_comments association, which contains only those comments that have been approved.

If you eager load an association with a specified :limit option, it will be ignored, returning all the associated objects:

class Picture < ActiveRecord::Base
  has_many :most_recent_comments, -> { order('id DESC').limit(10) }, class_name: 'Comment'
end

Picture.includes(:most_recent_comments).first.most_recent_comments # => returns all associated comments.

Eager loading is supported with polymorphic associations.

class Address < ActiveRecord::Base
  belongs_to :addressable, polymorphic: true
end

A call that tries to eager load the addressable model

Address.includes(:addressable)

This will execute one query to load the addresses and load the addressables with one query per addressable type. For example, if all the addressables are either of class Person or Company, then a total of 3 queries will be executed. The list of addressable types to load is determined on the back of the addresses loaded. This is not supported if Active Record has to fall back to the previous implementation of eager loading and will raise ActiveRecord::EagerLoadPolymorphicError. The reason is that the parent model’s type is a column value so its corresponding table name cannot be put in the FROM/JOIN clauses of that query.

Table Aliasing

Active Record uses table aliasing in the case that a table is referenced multiple times in a join. If a table is referenced only once, the standard table name is used. The second time, the table is aliased as #{reflection_name}_#{parent_table_name}. Indexes are appended for any more successive uses of the table name.

Post.joins(:comments)
# SELECT ... FROM posts INNER JOIN comments ON ...
Post.joins(:special_comments) # STI
# SELECT ... FROM posts INNER JOIN comments ON ... AND comments.type = 'SpecialComment'
Post.joins(:comments, :special_comments) # special_comments is the reflection name, posts is the parent table name
# SELECT ... FROM posts INNER JOIN comments ON ... INNER JOIN comments special_comments_posts

Acts as tree example:

TreeMixin.joins(:children)
# SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
TreeMixin.joins(children: :parent)
# SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
#                        INNER JOIN parents_mixins ...
TreeMixin.joins(children: {parent: :children})
# SELECT ... FROM mixins INNER JOIN mixins childrens_mixins ...
#                        INNER JOIN parents_mixins ...
#                        INNER JOIN mixins childrens_mixins_2

Has and Belongs to Many join tables use the same idea, but add a _join suffix:

Post.joins(:categories)
# SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
Post.joins(categories: :posts)
# SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
#                       INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
Post.joins(categories: {posts: :categories})
# SELECT ... FROM posts INNER JOIN categories_posts ... INNER JOIN categories ...
#                       INNER JOIN categories_posts posts_categories_join INNER JOIN posts posts_categories
#                       INNER JOIN categories_posts categories_posts_join INNER JOIN categories categories_posts_2

If you wish to specify your own custom joins using ActiveRecord::QueryMethods#joins method, those table names will take precedence over the eager associations:

Post.joins(:comments).joins("inner join comments ...")
# SELECT ... FROM posts INNER JOIN comments_posts ON ... INNER JOIN comments ...
Post.joins(:comments, :special_comments).joins("inner join comments ...")
# SELECT ... FROM posts INNER JOIN comments comments_posts ON ...
#                       INNER JOIN comments special_comments_posts ...
#                       INNER JOIN comments ...

Table aliases are automatically truncated according to the maximum length of table identifiers according to the specific database.

Modules

By default, associations will look for objects within the current module scope. Consider:

module MyApplication
  module Business
    class Firm < ActiveRecord::Base
      has_many :clients
    end

    class Client < ActiveRecord::Base; end
  end
end

When Firm#clients is called, it will in turn call MyApplication::Business::Client.find_all_by_firm_id(firm.id). If you want to associate with a class in another module scope, this can be done by specifying the complete class name.

module MyApplication
  module Business
    class Firm < ActiveRecord::Base; end
  end

  module Billing
    class Account < ActiveRecord::Base
      belongs_to :firm, class_name: "MyApplication::Business::Firm"
    end
  end
end

Bi-directional associations

When you specify an association, there is usually an association on the associated model that specifies the same relationship in reverse. For example, with the following models:

class Dungeon < ActiveRecord::Base
  has_many :traps
  has_one :evil_wizard
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon
end

class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon
end

The traps association on Dungeon and the dungeon association on Trap are the inverse of each other, and the inverse of the dungeon association on EvilWizard is the evil_wizard association on Dungeon (and vice-versa). By default, Active Record can guess the inverse of the association based on the name of the class. The result is the following:

d = Dungeon.first
t = d.traps.first
d.object_id == t.dungeon.object_id # => true

The Dungeon instances d and t.dungeon in the above example refer to the same in-memory instance since the association matches the name of the class. The result would be the same if we added :inverse_of to our model definitions:

class Dungeon < ActiveRecord::Base
  has_many :traps, inverse_of: :dungeon
  has_one :evil_wizard, inverse_of: :dungeon
end

class Trap < ActiveRecord::Base
  belongs_to :dungeon, inverse_of: :traps
end

class EvilWizard < ActiveRecord::Base
  belongs_to :dungeon, inverse_of: :evil_wizard
end

For more information, see the documentation for the :inverse_of option and the Active Record Associations guide.

Deleting from associations

Dependent associations

has_many, has_one, and belongs_to associations support the :dependent option. This allows you to specify that associated records should be deleted when the owner is deleted.

For example:

class Author
  has_many :posts, dependent: :destroy
end
Author.find(1).destroy # => Will destroy all of the author's posts, too

The :dependent option can have different values which specify how the deletion is done. For more information, see the documentation for this option on the different specific association types. When no option is given, the behavior is to do nothing with the associated records when destroying a record.

Note that :dependent is implemented using Rails’ callback system, which works by processing callbacks in order. Therefore, other callbacks declared either before or after the :dependent option can affect what it does.

Note that :dependent option is ignored for has_one :through associations.

Delete or destroy?

has_many and has_and_belongs_to_many associations have the methods destroy, delete, destroy_all and delete_all.

For has_and_belongs_to_many, delete and destroy are the same: they cause the records in the join table to be removed.

For has_many, destroy and destroy_all will always call the destroy method of the record(s) being removed so that callbacks are run. However delete and delete_all will either do the deletion according to the strategy specified by the :dependent option, or if no :dependent option is given, then it will follow the default strategy. The default strategy is to do nothing (leave the foreign keys with the parent ids set), except for has_many :through, where the default strategy is delete_all (delete the join records, without running their callbacks).

There is also a clear method which is the same as delete_all, except that it returns the association rather than the records which have been deleted.

What gets deleted?

There is a potential pitfall here: has_and_belongs_to_many and has_many :through associations have records in join tables, as well as the associated records. So when we call one of these deletion methods, what exactly should be deleted?

The answer is that it is assumed that deletion on an association is about removing the link between the owner and the associated object(s), rather than necessarily the associated objects themselves. So with has_and_belongs_to_many and has_many :through, the join records will be deleted, but the associated records won’t.

This makes sense if you think about it: if you were to call post.tags.delete(Tag.find_by(name: 'food')) you would want the ‘food’ tag to be unlinked from the post, rather than for the tag itself to be removed from the database.

However, there are examples where this strategy doesn’t make sense. For example, suppose a person has many projects, and each project has many tasks. If we deleted one of a person’s tasks, we would probably not want the project to be deleted. In this scenario, the delete method won’t actually work: it can only be used if the association on the join model is a belongs_to. In other situations you are expected to perform operations directly on either the associated records or the :through association.

With a regular has_many there is no distinction between the “associated records” and the “link”, so there is only one choice for what gets deleted.

With has_and_belongs_to_many and has_many :through, if you want to delete the associated records themselves, you can always do something along the lines of person.tasks.each(&:destroy).

Type safety with ActiveRecord::AssociationTypeMismatch

If you attempt to assign an object to an association that doesn’t match the inferred or specified :class_name, you’ll get an ActiveRecord::AssociationTypeMismatch.

Options

All of the association macros can be specialized through options. This makes cases more complex than the simple and guessable ones possible.

Methods

Instance Public methods

belongs_to(name, scope = nil, **options)

Specifies a one-to-one association with another class. This method should only be used if this class contains the foreign key. If the other class contains the foreign key, then you should use has_one instead. See Is it a belongs_to or has_one association? for more detail on when to use has_one and when to use belongs_to.

Methods will be added for retrieval and query for a single associated object, for which this object holds an id:

association is a placeholder for the symbol passed as the name argument, so belongs_to :author would add among others author.nil?.

association

Returns the associated object. nil is returned if none is found.

association=(associate)

Assigns the associate object, extracts the primary key, and sets it as the foreign key. No modification or deletion of existing records takes place.

build_association(attributes = {})

Returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key, but has not yet been saved.

create_association(attributes = {})

Returns a new object of the associated type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

create_association!(attributes = {})

Does the same as create_association, but raises ActiveRecord::RecordInvalid if the record is invalid.

reload_association

Returns the associated object, forcing a database read.

reset_association

Unloads the associated object. The next access will query it from the database.

association_changed?

Returns true if a new associate object has been assigned and the next save will update the foreign key.

association_previously_changed?

Returns true if the previous save updated the association to reference a new associate object.

Example

class Post < ActiveRecord::Base
  belongs_to :author
end

Declaring belongs_to :author adds the following methods (and more):

post = Post.find(7)
author = Author.find(19)

post.author           # similar to Author.find(post.author_id)
post.author = author  # similar to post.author_id = author.id
post.build_author     # similar to post.author = Author.new
post.create_author    # similar to post.author = Author.new; post.author.save; post.author
post.create_author!   # similar to post.author = Author.new; post.author.save!; post.author
post.reload_author
post.reset_author
post.author_changed?
post.author_previously_changed?

Scopes

You can pass a second argument scope as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.

Scope examples:

belongs_to :firm, -> { where(id: 2) }
belongs_to :user, -> { joins(:friends) }
belongs_to :level, ->(game) { where("game_level > ?", game.current_level) }

Options

The declaration can also include an options hash to specialize the behavior of the association.

:class_name

Specify the class name of the association. Use it only if that name can’t be inferred from the association name. So belongs_to :author will by default be linked to the Author class, but if the real class name is Person, you’ll have to specify it with this option.

:foreign_key

Specify the foreign key used for the association. By default this is guessed to be the name of the association with an “_id” suffix. So a class that defines a belongs_to :person association will use “person_id” as the default :foreign_key. Similarly, belongs_to :favorite_person, class_name: "Person" will use a foreign key of “favorite_person_id”.

Setting the :foreign_key option prevents automatic detection of the association’s inverse, so it is generally a good idea to set the :inverse_of option as well.

:foreign_type

Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the association with a “_type” suffix. So a class that defines a belongs_to :taggable, polymorphic: true association will use “taggable_type” as the default :foreign_type.

:primary_key

Specify the method that returns the primary key of associated object used for the association. By default this is id.

:dependent

If set to :destroy, the associated object is destroyed when this object is. If set to :delete, the associated object is deleted without calling its destroy method. If set to :destroy_async, the associated object is scheduled to be destroyed in a background job. This option should not be specified when belongs_to is used in conjunction with a has_many relationship on another class because of the potential to leave orphaned records behind.

:counter_cache

Caches the number of belonging objects on the associate class through the use of CounterCache::ClassMethods#increment_counter and CounterCache::ClassMethods#decrement_counter. The counter cache is incremented when an object of this class is created and decremented when it’s destroyed. This requires that a column named #{table_name}_count (such as comments_count for a belonging Comment class) is used on the associate class (such as a Post class) - that is the migration for #{table_name}_count is created on the associate class (such that Post.comments_count will return the count cached). You can also specify a custom counter cache column by providing a column name instead of a true/false value to this option (e.g., counter_cache: :my_custom_counter.)

Starting to use counter caches on existing large tables can be troublesome, because the column values must be backfilled separately of the column addition (to not lock the table for too long) and before the use of :counter_cache (otherwise methods like size/any?/etc, which use counter caches internally, can produce incorrect results). To safely backfill the values while keeping counter cache columns updated with the child records creation/removal and to avoid the mentioned methods use the possibly incorrect counter cache column values and always get the results from the database, use counter_cache: { active: false }. If you also need to specify a custom column name, use counter_cache: { active: false, column: :my_custom_counter }.

Note: If you’ve enabled the counter cache, then you may want to add the counter cache attribute to the attr_readonly list in the associated classes (e.g. class Post; attr_readonly :comments_count; end).

:polymorphic

Specify this association is a polymorphic association by passing true. Note: Since polymorphic associations rely on storing class names in the database, make sure to update the class names in the *_type polymorphic type column of the corresponding rows.

:validate

When set to true, validates new objects added to association when saving the parent object. false by default. If you want to ensure associated objects are revalidated on every update, use validates_associated.

:autosave

If true, always save the associated object or destroy it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object. By default, only save the associated object if it’s a new record.

Note that NestedAttributes::ClassMethods#accepts_nested_attributes_for sets :autosave to true.

:touch

If true, the associated object will be touched (the updated_at / updated_on attributes set to current time) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time in addition to the updated_at / updated_on attribute. Please note that no validation will be performed when touching, and only the after_touch, after_commit, and after_rollback callbacks will be executed.

:inverse_of

Specifies the name of the has_one or has_many association on the associated object that is the inverse of this belongs_to association. See Bi-directional associations for more detail.

:optional

When set to true, the association will not have its presence validated.

:required

When set to true, the association will also have its presence validated. This will validate the association itself, not the id. You can use :inverse_of to avoid an extra query during validation. NOTE: required is set to true by default and is deprecated. If you don’t want to have association presence validated, use optional: true.

:default

Provide a callable (i.e. proc or lambda) to specify that the association should be initialized with a particular record before validation. Please note that callable won’t be executed if the record exists.

:strict_loading

Enforces strict loading every time the associated record is loaded through this association.

:ensuring_owner_was

Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.

:query_constraints

Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or query_constraints size.

Option examples:

belongs_to :firm, foreign_key: "client_of"
belongs_to :person, primary_key: "name", foreign_key: "person_name"
belongs_to :author, class_name: "Person", foreign_key: "author_id"
belongs_to :valid_coupon, ->(o) { where "discounts > ?", o.payments_count },
                          class_name: "Coupon", foreign_key: "coupon_id"
belongs_to :attachable, polymorphic: true
belongs_to :project, -> { readonly }
belongs_to :post, counter_cache: true
belongs_to :comment, touch: true
belongs_to :company, touch: :employees_last_updated_at
belongs_to :user, optional: true
belongs_to :account, default: -> { company.account }
belongs_to :account, strict_loading: true
belongs_to :note, query_constraints: [:organization_id, :note_id]
📝 Source code 
# File activerecord/lib/active_record/associations.rb, line 1659
        def belongs_to(name, scope = nil, **options)
          reflection = Builder::BelongsTo.build(self, name, scope, options)
          Reflection.add_reflection self, name, reflection
        end
🔎 See on GitHub

has_and_belongs_to_many(name, scope = nil, **options, &extension)

Specifies a many-to-many relationship with another class. This associates two classes via an intermediate join table. Unless the join table is explicitly specified as an option, it is guessed using the lexical order of the class names. So a join between Developer and Project will give the default join table name of “developers_projects” because “D” precedes “P” alphabetically. Note that this precedence is calculated using the < operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables “paper_boxes” and “papers” to generate a join table name of “papers_paper_boxes” because of the length of the name “paper_boxes”, but it in fact generates a join table name of “paper_boxes_papers”. Be aware of this caveat, and use the custom :join_table option if you need to. If your tables share a common prefix, it will only appear once at the beginning. For example, the tables “catalog_categories” and “catalog_products” generate a join table name of “catalog_categories_products”.

The join table should not have a primary key or a model associated with it. You must manually generate the join table with a migration such as this:

class CreateDevelopersProjectsJoinTable < ActiveRecord::Migration[7.2]
  def change
    create_join_table :developers, :projects
  end
end

It’s also a good idea to add indexes to each of those columns to speed up the joins process. However, in MySQL it is advised to add a compound index for both of the columns as MySQL only uses one index per table during the lookup.

Adds the following methods for retrieval and query:

collection is a placeholder for the symbol passed as the name argument, so has_and_belongs_to_many :categories would add among others categories.empty?.

collection

Returns a Relation of all the associated objects. An empty Relation is returned if none are found.

collection<<(object, ...)

Adds one or more objects to the collection by creating associations in the join table (collection.push and collection.concat are aliases to this method). Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record.

collection.delete(object, ...)

Removes one or more objects from the collection by removing their associations from the join table. This does not destroy the objects.

collection.destroy(object, ...)

Removes one or more objects from the collection by running destroy on each association in the join table, overriding any dependent option. This does not destroy the objects.

collection=objects

Replaces the collection’s content by deleting and adding objects as appropriate.

collection_singular_ids

Returns an array of the associated objects’ ids.

collection_singular_ids=ids

Replace the collection by the objects identified by the primary keys in ids.

collection.clear

Removes every object from the collection. This does not destroy the objects.

collection.empty?

Returns true if there are no associated objects.

collection.size

Returns the number of associated objects.

collection.find(id)

Finds an associated object responding to the id and that meets the condition that it has to be associated with this object. Uses the same rules as ActiveRecord::FinderMethods#find.

collection.exists?(...)

Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::FinderMethods#exists?.

collection.build(attributes = {})

Returns a new object of the collection type that has been instantiated with attributes and linked to this object through the join table, but has not yet been saved.

collection.create(attributes = {})

Returns a new object of the collection type that has been instantiated with attributes, linked to this object through the join table, and that has already been saved (if it passed the validation).

collection.reload

Returns a Relation of all of the associated objects, forcing a database read. An empty Relation is returned if none are found.

Example

class Developer < ActiveRecord::Base
  has_and_belongs_to_many :projects
end

Declaring has_and_belongs_to_many :projects adds the following methods (and more):

developer = Developer.find(11)
project   = Project.find(9)

developer.projects
developer.projects << project
developer.projects.delete(project)
developer.projects.destroy(project)
developer.projects = [project]
developer.project_ids
developer.project_ids = [9]
developer.projects.clear
developer.projects.empty?
developer.projects.size
developer.projects.find(9)
developer.projects.exists?(9)
developer.projects.build  # similar to Project.new(developer_id: 11)
developer.projects.create # similar to Project.create(developer_id: 11)
developer.projects.reload

The declaration may include an options hash to specialize the behavior of the association.

Scopes

You can pass a second argument scope as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.

Scope examples:

has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
has_and_belongs_to_many :categories, ->(post) {
  where("default_category = ?", post.default_category)
}

Extensions

The extension argument allows you to pass a block into a has_and_belongs_to_many association. This is useful for adding new finders, creators, and other factory-type methods to be used as part of the association.

Extension examples:

has_and_belongs_to_many :contractors do
  def find_or_create_by_name(name)
    first_name, last_name = name.split(" ", 2)
    find_or_create_by(first_name: first_name, last_name: last_name)
  end
end

Options

:class_name

Specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_and_belongs_to_many :projects will by default be linked to the Project class, but if the real class name is SuperProject, you’ll have to specify it with this option.

:join_table

Specify the name of the join table if the default based on lexical order isn’t what you want. WARNING: If you’re overwriting the table name of either class, the table_name method MUST be declared underneath any has_and_belongs_to_many declaration in order to work.

:foreign_key

Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_and_belongs_to_many association to Project will use “person_id” as the default :foreign_key.

Setting the :foreign_key option prevents automatic detection of the association’s inverse, so it is generally a good idea to set the :inverse_of option as well.

:association_foreign_key

Specify the foreign key used for the association on the receiving side of the association. By default this is guessed to be the name of the associated class in lower-case and “_id” suffixed. So if a Person class makes a has_and_belongs_to_many association to Project, the association will use “project_id” as the default :association_foreign_key.

:validate

When set to true, validates new objects added to association when saving the parent object. true by default. If you want to ensure associated objects are revalidated on every update, use validates_associated.

:autosave

If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records.

Note that NestedAttributes::ClassMethods#accepts_nested_attributes_for sets :autosave to true.

:strict_loading

Enforces strict loading every time an associated record is loaded through this association.

Option examples:

has_and_belongs_to_many :projects
has_and_belongs_to_many :projects, -> { includes(:milestones, :manager) }
has_and_belongs_to_many :nations, class_name: "Country"
has_and_belongs_to_many :categories, join_table: "prods_cats"
has_and_belongs_to_many :categories, -> { readonly }
has_and_belongs_to_many :categories, strict_loading: true
📝 Source code 
# File activerecord/lib/active_record/associations.rb, line 1840
        def has_and_belongs_to_many(name, scope = nil, **options, &extension)
          habtm_reflection = ActiveRecord::Reflection::HasAndBelongsToManyReflection.new(name, scope, options, self)

          builder = Builder::HasAndBelongsToMany.new name, self, options

          join_model = builder.through_model

          const_set join_model.name, join_model
          private_constant join_model.name

          middle_reflection = builder.middle_reflection join_model

          Builder::HasMany.define_callbacks self, middle_reflection
          Reflection.add_reflection self, middle_reflection.name, middle_reflection
          middle_reflection.parent_reflection = habtm_reflection

          include Module.new {
            class_eval <<-RUBY, __FILE__, __LINE__ + 1
              def destroy_associations
                association(:#{middle_reflection.name}).delete_all(:delete_all)
                association(:#{name}).reset
                super
              end
            RUBY
          }

          hm_options = {}
          hm_options[:through] = middle_reflection.name
          hm_options[:source] = join_model.right_reflection.name

          [:before_add, :after_add, :before_remove, :after_remove, :autosave, :validate, :join_table, :class_name, :extend, :strict_loading].each do |k|
            hm_options[k] = options[k] if options.key? k
          end

          has_many name, scope, **hm_options, &extension
          _reflections[name].parent_reflection = habtm_reflection
        end
🔎 See on GitHub

has_many(name, scope = nil, **options, &extension)

Specifies a one-to-many association. The following methods for retrieval and query of collections of associated objects will be added:

collection is a placeholder for the symbol passed as the name argument, so has_many :clients would add among others clients.empty?.

collection

Returns a Relation of all the associated objects. An empty Relation is returned if none are found.

collection<<(object, ...)

Adds one or more objects to the collection by setting their foreign keys to the collection’s primary key. Note that this operation instantly fires update SQL without waiting for the save or update call on the parent object, unless the parent object is a new record. This will also run validations and callbacks of associated object(s).

collection.delete(object, ...)

Removes one or more objects from the collection by setting their foreign keys to NULL. Objects will be in addition destroyed if they’re associated with dependent: :destroy, and deleted if they’re associated with dependent: :delete_all.

If the :through option is used, then the join records are deleted (rather than nullified) by default, but you can specify dependent: :destroy or dependent: :nullify to override this.

collection.destroy(object, ...)

Removes one or more objects from the collection by running destroy on each record, regardless of any dependent option, ensuring callbacks are run.

If the :through option is used, then the join records are destroyed instead, not the objects themselves.

collection=objects

Replaces the collections content by deleting and adding objects as appropriate. If the :through option is true callbacks in the join models are triggered except destroy callbacks, since deletion is direct by default. You can specify dependent: :destroy or dependent: :nullify to override this.

collection_singular_ids

Returns an array of the associated objects’ ids

collection_singular_ids=ids

Replace the collection with the objects identified by the primary keys in ids. This method loads the models and calls collection=. See above.

collection.clear

Removes every object from the collection. This destroys the associated objects if they are associated with dependent: :destroy, deletes them directly from the database if dependent: :delete_all, otherwise sets their foreign keys to NULL. If the :through option is true no destroy callbacks are invoked on the join models. Join models are directly deleted.

collection.empty?

Returns true if there are no associated objects.

collection.size

Returns the number of associated objects.

collection.find(...)

Finds an associated object according to the same rules as ActiveRecord::FinderMethods#find.

collection.exists?(...)

Checks whether an associated object with the given conditions exists. Uses the same rules as ActiveRecord::FinderMethods#exists?.

collection.build(attributes = {}, ...)

Returns one or more new objects of the collection type that have been instantiated with attributes and linked to this object through a foreign key, but have not yet been saved.

collection.create(attributes = {})

Returns a new object of the collection type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation). Note: This only works if the base model already exists in the DB, not if it is a new (unsaved) record!

collection.create!(attributes = {})

Does the same as collection.create, but raises ActiveRecord::RecordInvalid if the record is invalid.

collection.reload

Returns a Relation of all of the associated objects, forcing a database read. An empty Relation is returned if none are found.

Example

class Firm < ActiveRecord::Base
  has_many :clients
end

Declaring has_many :clients adds the following methods (and more):

firm = Firm.find(2)
client = Client.find(6)

firm.clients                       # similar to Client.where(firm_id: 2)
firm.clients << client
firm.clients.delete(client)
firm.clients.destroy(client)
firm.clients = [client]
firm.client_ids
firm.client_ids = [6]
firm.clients.clear
firm.clients.empty?                # similar to firm.clients.size == 0
firm.clients.size                  # similar to Client.count "firm_id = 2"
firm.clients.find                  # similar to Client.where(firm_id: 2).find(6)
firm.clients.exists?(name: 'ACME') # similar to Client.exists?(name: 'ACME', firm_id: 2)
firm.clients.build                 # similar to Client.new(firm_id: 2)
firm.clients.create                # similar to Client.create(firm_id: 2)
firm.clients.create!               # similar to Client.create!(firm_id: 2)
firm.clients.reload

The declaration can also include an options hash to specialize the behavior of the association.

Scopes

You can pass a second argument scope as a callable (i.e. proc or lambda) to retrieve a specific set of records or customize the generated query when you access the associated collection.

Scope examples:

has_many :comments, -> { where(author_id: 1) }
has_many :employees, -> { joins(:address) }
has_many :posts, ->(blog) { where("max_post_length > ?", blog.max_post_length) }

Extensions

The extension argument allows you to pass a block into a has_many association. This is useful for adding new finders, creators, and other factory-type methods to be used as part of the association.

Extension examples:

has_many :employees do
  def find_or_create_by_name(name)
    first_name, last_name = name.split(" ", 2)
    find_or_create_by(first_name: first_name, last_name: last_name)
  end
end

Options

:class_name

Specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_many :products will by default be linked to the Product class, but if the real class name is SpecialProduct, you’ll have to specify it with this option.

:foreign_key

Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_many association will use “person_id” as the default :foreign_key.

Setting the :foreign_key option prevents automatic detection of the association’s inverse, so it is generally a good idea to set the :inverse_of option as well.

:foreign_type

Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a has_many :tags, as: :taggable association will use “taggable_type” as the default :foreign_type.

:primary_key

Specify the name of the column to use as the primary key for the association. By default this is id.

:dependent

Controls what happens to the associated objects when their owner is destroyed. Note that these are implemented as callbacks, and Rails executes callbacks in order. Therefore, other similar callbacks may affect the :dependent behavior, and the :dependent behavior may affect other callbacks.

  • nil do nothing (default).

  • :destroy causes all the associated objects to also be destroyed.

  • :destroy_async destroys all the associated objects in a background job. WARNING: Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner.

  • :delete_all causes all the associated objects to be deleted directly from the database (so callbacks will not be executed).

  • :nullify causes the foreign keys to be set to NULL. Polymorphic type will also be nullified on polymorphic associations. Callbacks are not executed.

  • :restrict_with_exception causes an ActiveRecord::DeleteRestrictionError exception to be raised if there are any associated records.

  • :restrict_with_error causes an error to be added to the owner if there are any associated objects.

If using with the :through option, the association on the join model must be a belongs_to, and the records which get deleted are the join records, rather than the associated records.

If using dependent: :destroy on a scoped association, only the scoped objects are destroyed. For example, if a Post model defines has_many :comments, -> { where published: true }, dependent: :destroy and destroy is called on a post, only published comments are destroyed. This means that any unpublished comments in the database would still contain a foreign key pointing to the now deleted post.

:counter_cache

This option can be used to configure a custom named :counter_cache. You only need this option, when you customized the name of your :counter_cache on the belongs_to association.

:as

Specifies a polymorphic interface (See belongs_to).

:through

Specifies an association through which to perform the query. This can be any other type of association, including other :through associations. Options for :class_name, :primary_key and :foreign_key are ignored, as the association uses the source reflection.

If the association on the join model is a belongs_to, the collection can be modified and the records on the :through model will be automatically created and removed as appropriate. Otherwise, the collection is read-only, so you should manipulate the :through association directly.

If you are going to modify the association (rather than just read from it), then it is a good idea to set the :inverse_of option on the source association on the join model. This allows associated records to be built which will automatically create the appropriate join model records when they are saved. See Association Join Models and Setting Inverses for more detail.

:disable_joins

Specifies whether joins should be skipped for an association. If set to true, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on has_many :through associations as has_many alone do not perform a join.

:source

Specifies the source association name used by has_many :through queries. Only use it if the name cannot be inferred from the association. has_many :subscribers, through: :subscriptions will look for either :subscribers or :subscriber on Subscription, unless a :source is given.

:source_type

Specifies type of the source association used by has_many :through queries where the source association is a polymorphic belongs_to.

:validate

When set to true, validates new objects added to association when saving the parent object. true by default. If you want to ensure associated objects are revalidated on every update, use validates_associated.

:autosave

If true, always save the associated objects or destroy them if marked for destruction, when saving the parent object. If false, never save or destroy the associated objects. By default, only save associated objects that are new records. This option is implemented as a before_save callback. Because callbacks are run in the order they are defined, associated objects may need to be explicitly saved in any user-defined before_save callbacks.

Note that NestedAttributes::ClassMethods#accepts_nested_attributes_for sets :autosave to true.

:inverse_of

Specifies the name of the belongs_to association on the associated object that is the inverse of this has_many association. See Bi-directional associations for more detail.

:extend

Specifies a module or array of modules that will be extended into the association object returned. Useful for defining methods on associations, especially when they should be shared between multiple association objects.

:strict_loading

When set to true, enforces strict loading every time the associated record is loaded through this association.

:ensuring_owner_was

Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.

:query_constraints

Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or query_constraints size.

:index_errors

Allows differentiation of multiple validation errors from the association records, by including an index in the error attribute name, e.g. roles[2].level. When set to true, the index is based on association order, i.e. database order, with yet to be persisted new records placed at the end. When set to :nested_attributes_order, the index is based on the record order received by nested attributes setter, when accepts_nested_attributes_for is used.

Option examples:

has_many :comments, -> { order("posted_on") }
has_many :comments, -> { includes(:author) }
has_many :people, -> { where(deleted: false).order("name") }, class_name: "Person"
has_many :tracks, -> { order("position") }, dependent: :destroy
has_many :comments, dependent: :nullify
has_many :tags, as: :taggable
has_many :reports, -> { readonly }
has_many :subscribers, through: :subscriptions, source: :user
has_many :subscribers, through: :subscriptions, disable_joins: true
has_many :comments, strict_loading: true
has_many :comments, query_constraints: [:blog_id, :post_id]
has_many :comments, index_errors: :nested_attributes_order
📝 Source code 
# File activerecord/lib/active_record/associations.rb, line 1272
        def has_many(name, scope = nil, **options, &extension)
          reflection = Builder::HasMany.build(self, name, scope, options, &extension)
          Reflection.add_reflection self, name, reflection
        end
🔎 See on GitHub

has_one(name, scope = nil, **options)

Specifies a one-to-one association with another class. This method should only be used if the other class contains the foreign key. If the current class contains the foreign key, then you should use belongs_to instead. See Is it a belongs_to or has_one association? for more detail on when to use has_one and when to use belongs_to.

The following methods for retrieval and query of a single associated object will be added:

association is a placeholder for the symbol passed as the name argument, so has_one :manager would add among others manager.nil?.

association

Returns the associated object. nil is returned if none is found.

association=(associate)

Assigns the associate object, extracts the primary key, sets it as the foreign key, and saves the associate object. To avoid database inconsistencies, permanently deletes an existing associated object when assigning a new one, even if the new one isn’t saved to database.

build_association(attributes = {})

Returns a new object of the associated type that has been instantiated with attributes and linked to this object through a foreign key, but has not yet been saved.

create_association(attributes = {})

Returns a new object of the associated type that has been instantiated with attributes, linked to this object through a foreign key, and that has already been saved (if it passed the validation).

create_association!(attributes = {})

Does the same as create_association, but raises ActiveRecord::RecordInvalid if the record is invalid.

reload_association

Returns the associated object, forcing a database read.

reset_association

Unloads the associated object. The next access will query it from the database.

Example

class Account < ActiveRecord::Base
  has_one :beneficiary
end

Declaring has_one :beneficiary adds the following methods (and more):

account = Account.find(5)
beneficiary = Beneficiary.find(8)

account.beneficiary               # similar to Beneficiary.find_by(account_id: 5)
account.beneficiary = beneficiary # similar to beneficiary.update(account_id: 5)
account.build_beneficiary         # similar to Beneficiary.new(account_id: 5)
account.create_beneficiary        # similar to Beneficiary.create(account_id: 5)
account.create_beneficiary!       # similar to Beneficiary.create!(account_id: 5)
account.reload_beneficiary
account.reset_beneficiary

Scopes

You can pass a second argument scope as a callable (i.e. proc or lambda) to retrieve a specific record or customize the generated query when you access the associated object.

Scope examples:

has_one :author, -> { where(comment_id: 1) }
has_one :employer, -> { joins(:company) }
has_one :latest_post, ->(blog) { where("created_at > ?", blog.enabled_at) }

Options

The declaration can also include an options hash to specialize the behavior of the association.

Options are:

:class_name

Specify the class name of the association. Use it only if that name can’t be inferred from the association name. So has_one :manager will by default be linked to the Manager class, but if the real class name is Person, you’ll have to specify it with this option.

:dependent

Controls what happens to the associated object when its owner is destroyed:

  • nil do nothing (default).

  • :destroy causes the associated object to also be destroyed

  • :destroy_async causes the associated object to be destroyed in a background job. WARNING: Do not use this option if the association is backed by foreign key constraints in your database. The foreign key constraint actions will occur inside the same transaction that deletes its owner.

  • :delete causes the associated object to be deleted directly from the database (so callbacks will not execute)

  • :nullify causes the foreign key to be set to NULL. Polymorphic type column is also nullified on polymorphic associations. Callbacks are not executed.

  • :restrict_with_exception causes an ActiveRecord::DeleteRestrictionError exception to be raised if there is an associated record

  • :restrict_with_error causes an error to be added to the owner if there is an associated object

Note that :dependent option is ignored when using :through option.

:foreign_key

Specify the foreign key used for the association. By default this is guessed to be the name of this class in lower-case and “_id” suffixed. So a Person class that makes a has_one association will use “person_id” as the default :foreign_key.

Setting the :foreign_key option prevents automatic detection of the association’s inverse, so it is generally a good idea to set the :inverse_of option as well.

:foreign_type

Specify the column used to store the associated object’s type, if this is a polymorphic association. By default this is guessed to be the name of the polymorphic association specified on “as” option with a “_type” suffix. So a class that defines a has_one :tag, as: :taggable association will use “taggable_type” as the default :foreign_type.

:primary_key

Specify the method that returns the primary key used for the association. By default this is id.

:as

Specifies a polymorphic interface (See belongs_to).

:through

Specifies a Join Model through which to perform the query. Options for :class_name, :primary_key, and :foreign_key are ignored, as the association uses the source reflection. You can only use a :through query through a has_one or belongs_to association on the join model.

If the association on the join model is a belongs_to, the collection can be modified and the records on the :through model will be automatically created and removed as appropriate. Otherwise, the collection is read-only, so you should manipulate the :through association directly.

If you are going to modify the association (rather than just read from it), then it is a good idea to set the :inverse_of option on the source association on the join model. This allows associated records to be built which will automatically create the appropriate join model records when they are saved. See Association Join Models and Setting Inverses for more detail.

:disable_joins

Specifies whether joins should be skipped for an association. If set to true, two or more queries will be generated. Note that in some cases, if order or limit is applied, it will be done in-memory due to database limitations. This option is only applicable on has_one :through associations as has_one alone does not perform a join.

:source

Specifies the source association name used by has_one :through queries. Only use it if the name cannot be inferred from the association. has_one :favorite, through: :favorites will look for a :favorite on Favorite, unless a :source is given.

:source_type

Specifies type of the source association used by has_one :through queries where the source association is a polymorphic belongs_to.

:validate

When set to true, validates new objects added to association when saving the parent object. false by default. If you want to ensure associated objects are revalidated on every update, use validates_associated.

:autosave

If true, always saves the associated object or destroys it if marked for destruction, when saving the parent object. If false, never save or destroy the associated object.

By default, only saves the associated object if it’s a new record. Setting this option to true also enables validations on the associated object unless explicitly disabled with validate: false. This is because saving an object with invalid associated objects would fail, so any associated objects will go through validation checks.

Note that NestedAttributes::ClassMethods#accepts_nested_attributes_for sets :autosave to true.

:touch

If true, the associated object will be touched (the updated_at / updated_on attributes set to current time) when this record is either saved or destroyed. If you specify a symbol, that attribute will be updated with the current time in addition to the updated_at / updated_on attribute. Please note that no validation will be performed when touching, and only the after_touch, after_commit, and after_rollback callbacks will be executed.

:inverse_of

Specifies the name of the belongs_to association on the associated object that is the inverse of this has_one association. See Bi-directional associations for more detail.

:required

When set to true, the association will also have its presence validated. This will validate the association itself, not the id. You can use :inverse_of to avoid an extra query during validation.

:strict_loading

Enforces strict loading every time the associated record is loaded through this association.

:ensuring_owner_was

Specifies an instance method to be called on the owner. The method must return true in order for the associated records to be deleted in a background job.

:query_constraints

Serves as a composite foreign key. Defines the list of columns to be used to query the associated object. This is an optional option. By default Rails will attempt to derive the value automatically. When the value is set the Array size must match associated model’s primary key or query_constraints size.

Option examples:

has_one :credit_card, dependent: :destroy  # destroys the associated credit card
has_one :credit_card, dependent: :nullify  # updates the associated records foreign
                                              # key value to NULL rather than destroying it
has_one :last_comment, -> { order('posted_on') }, class_name: "Comment"
has_one :project_manager, -> { where(role: 'project_manager') }, class_name: "Person"
has_one :attachment, as: :attachable
has_one :boss, -> { readonly }
has_one :club, through: :membership
has_one :club, through: :membership, disable_joins: true
has_one :primary_address, -> { where(primary: true) }, through: :addressables, source: :addressable
has_one :credit_card, required: true
has_one :credit_card, strict_loading: true
has_one :employment_record_book, query_constraints: [:organization_id, :employee_id]
📝 Source code 
# File activerecord/lib/active_record/associations.rb, line 1468
        def has_one(name, scope = nil, **options)
          reflection = Builder::HasOne.build(self, name, scope, options)
          Reflection.add_reflection self, name, reflection
        end
🔎 See on GitHub