ActiveRecord::DelegatedType

Delegated types

Class hierarchies can map to relational database tables in many ways. Active Record, for example, offers purely abstract classes, where the superclass doesn't persist any attributes, and single-table inheritance, where all attributes from all levels of the hierarchy are represented in a single table. Both have their places, but neither are without their drawbacks.

The problem with purely abstract classes is that all concrete subclasses must persist all the shared attributes themselves in their own tables (also known as class-table inheritance). This makes it hard to do queries across the hierarchy. For example, imagine you have the following hierarchy:

Entry < ApplicationRecord
Message < Entry
Comment < Entry

How do you show a feed that has both Message and Comment records, which can be easily paginated? Well, you can't! Messages are backed by a messages table and comments by a comments table. You can't pull from both tables at once and use a consistent OFFSET/LIMIT scheme.

You can get around the pagination problem by using single-table inheritance, but now you're forced into a single mega table with all the attributes from all subclasses. No matter how divergent. If a Message has a subject, but the comment does not, well, now the comment does anyway! So STI works best when there's little divergence between the subclasses and their attributes.

But there's a third way: Delegated types. With this approach, the “superclass” is a concrete class that is represented by its own table, where all the superclass attributes that are shared amongst all the “subclasses” are stored. And then each of the subclasses have their own individual tables for additional attributes that are particular to their implementation. This is similar to what's called multi-table inheritance in Django, but instead of actual inheritance, this approach uses delegation to form the hierarchy and share responsibilities.

Let's look at that entry/message/comment example using delegated types:

# Schema: entries[ id, account_id, creator_id, created_at, updated_at, entryable_type, entryable_id ]
class Entry < ApplicationRecord
  belongs_to :account
  belongs_to :creator
  delegated_type :entryable, types: %w[ Message Comment ]
end

module Entryable
  extend ActiveSupport::Concern

  included do
    has_one :entry, as: :entryable, touch: true
  end
end

# Schema: messages[ id, subject ]
class Message < ApplicationRecord
  include Entryable
  has_rich_text :content
end

# Schema: comments[ id, content ]
class Comment < ApplicationRecord
  include Entryable
end

As you can see, neither Message nor Comment are meant to stand alone. Crucial metadata for both classes resides in the Entry “superclass”. But the Entry absolutely can stand alone in terms of querying capacity in particular. You can now easily do things like:

Account.entries.order(created_at: :desc).limit(50)

Which is exactly what you want when displaying both comments and messages together. The entry itself can be rendered as its delegated type easily, like so:

# entries/_entry.html.erb
<%= render "entries/entryables/#{entry.entryable_name}", entry: entry %>

# entries/entryables/_message.html.erb
<div class="message">
  Posted on <%= entry.created_at %> by <%= entry.creator.name %>: <%= entry.message.content %>
</div>

# entries/entryables/_comment.html.erb
<div class="comment">
  <%= entry.creator.name %> said: <%= entry.comment.content %>
</div>

Sharing behavior with concerns and controllers

The entry “superclass” also serves as a perfect place to put all that shared logic that applies to both messages and comments, and which acts primarily on the shared attributes. Imagine:

class Entry < ApplicationRecord
  include Eventable, Forwardable, Redeliverable
end

Which allows you to have controllers for things like ForwardsController and RedeliverableController that both act on entries, and thus provide the shared functionality to both messages and comments.

Creating new records

You create a new record that uses delegated typing by creating the delegator and delegatee at the same time, like so:

Entry.create! entryable: Comment.new(content: "Hello!"), creator: Current.user

If you need more complicated composition, or you need to perform dependent validation, you should build a factory method or class to take care of the complicated needs. This could be as simple as:

class Entry < ApplicationRecord
  def self.create_with_comment(content, creator: Current.user)
    create! entryable: Comment.new(content: content), creator: creator
  end
end

Adding further delegation

The delegated type shouldn't just answer the question of what the underlying class is called. In fact, that's an anti-pattern most of the time. The reason you're building this hierarchy is to take advantage of polymorphism. So here's a simple example of that:

class Entry < ApplicationRecord
  delegated_type :entryable, types: %w[ Message Comment ]
  delegate :title, to: :entryable
end

class Message < ApplicationRecord
  def title
    subject
  end
end

class Comment < ApplicationRecord
  def title
    content.truncate(20)
  end
end

Now you can list a bunch of entries, call +Entry#title+, and polymorphism will provide you with the answer.