Active Record Migrations

Migrations can manage the evolution of a schema used by several physical databases. It's a solution to the common problem of adding a field to make a new feature work in your local database, but being unsure of how to push that change to other developers and to the production server. With migrations, you can describe the transformations in self-contained classes that can be checked into version control systems and executed against another database that might be one, two, or five versions behind.

Example of a simple migration:

class AddSsl < ActiveRecord::Migration[6.0]
  def up
    add_column :accounts, :ssl_enabled, :boolean, default: true
  end

  def down
    remove_column :accounts, :ssl_enabled
  end
end

This migration will add a boolean flag to the accounts table and remove it if you're backing out of the migration. It shows how all migrations have two methods up and down that describes the transformations required to implement or remove the migration. These methods can consist of both the migration specific methods like add_column and remove_column, but may also contain regular Ruby code for generating data needed for the transformations.

Example of a more complex migration that also needs to initialize data:

class AddSystemSettings < ActiveRecord::Migration[6.0]
  def up
    create_table :system_settings do |t|
      t.string  :name
      t.string  :label
      t.text    :value
      t.string  :type
      t.integer :position
    end

    SystemSetting.create  name:  'notice',
                          label: 'Use notice?',
                          value: 1
  end

  def down
    drop_table :system_settings
  end
end

This migration first adds the system_settings table, then creates the very first row in it using the Active Record model that relies on the table. It also uses the more advanced create_table syntax where you can specify a complete table schema in one block call.

Available transformations

Creation

• create_join_table(table_1, table_2, options): Creates a join table having its name as the lexical order of the first two arguments. See ActiveRecord::ConnectionAdapters::SchemaStatements#create_join_table for details.

• create_table(name, options): Creates a table called name and makes the table object available to a block that can then add columns to it, following the same format as add_column. See example above. The options hash is for fragments like “DEFAULT CHARSET=UTF-8” that are appended to the create table definition.

• add_column(table_name, column_name, type, options): Adds a new column to the table called table_name named column_name specified to be one of the following types: :string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, :boolean. A default value can be specified by passing an options hash like { default: 11 }. Other options include :limit and :null (e.g. { limit: 50, null: false }) – see ActiveRecord::ConnectionAdapters::TableDefinition#column for details.

• add_foreign_key(from_table, to_table, options): Adds a new foreign key. from_table is the table with the key column, to_table contains the referenced primary key.

• add_index(table_name, column_names, options): Adds a new index with the name of the column. Other options include :name, :unique (e.g. { name: 'users_name_index', unique: true }) and :order (e.g. { order: { name: :desc } }).

• add_reference(:table_name, :reference_name): Adds a new column reference_name_id by default an integer. See ActiveRecord::ConnectionAdapters::SchemaStatements#add_reference for details.

• add_timestamps(table_name, options): Adds timestamps (created_at and updated_at) columns to table_name.

Modification

• change_column(table_name, column_name, type, options): Changes the column to a different type using the same parameters as add_column.

• change_column_default(table_name, column_name, default_or_changes): Sets a default value for column_name defined by default_or_changes on table_name. Passing a hash containing :from and :to as default_or_changes will make this change reversible in the migration.

• change_column_null(table_name, column_name, null, default = nil): Sets or removes a NOT NULL constraint on column_name. The null flag indicates whether the value can be NULL. See ActiveRecord::ConnectionAdapters::SchemaStatements#change_column_null for details.

• change_table(name, options): Allows to make column alterations to the table called name. It makes the table object available to a block that can then add/remove columns, indexes or foreign keys to it.

• rename_column(table_name, column_name, new_column_name): Renames a column but keeps the type and content.

• rename_index(table_name, old_name, new_name): Renames an index.

• rename_table(old_name, new_name): Renames the table called old_name to new_name.

Deletion

• drop_table(name): Drops the table called name.

• drop_join_table(table_1, table_2, options): Drops the join table specified by the given arguments.

• remove_column(table_name, column_name, type, options): Removes the column named column_name from the table called table_name.

• remove_columns(table_name, *column_names): Removes the given columns from the table definition.

• remove_foreign_key(from_table, to_table = nil, **options): Removes the given foreign key from the table called table_name.

• remove_index(table_name, column: column_names): Removes the index specified by column_names.

• remove_index(table_name, name: index_name): Removes the index specified by index_name.

• remove_reference(table_name, ref_name, options): Removes the reference(s) on table_name specified by ref_name.

• remove_timestamps(table_name, options): Removes the timestamp columns (created_at and updated_at) from the table definition.

Irreversible transformations

Some transformations are destructive in a manner that cannot be reversed. Migrations of that kind should raise an ActiveRecord::IrreversibleMigration exception in their down method.

Running migrations from within Rails

The Rails package has several tools to help create and apply migrations.

To generate a new migration, you can use

bin/rails generate migration MyNewMigration

where MyNewMigration is the name of your migration. The generator will create an empty migration file timestamp_my_new_migration.rb in the db/migrate/ directory where timestamp is the UTC formatted date and time that the migration was generated.

There is a special syntactic shortcut to generate migrations that add fields to a table.

bin/rails generate migration add_fieldname_to_tablename fieldname:string

This will generate the file timestamp_add_fieldname_to_tablename.rb, which will look like this:

class AddFieldnameToTablename < ActiveRecord::Migration[6.0]
  def change
    add_column :tablenames, :fieldname, :string
  end
end

To run migrations against the currently configured database, use bin/rails db:migrate. This will update the database by running all of the pending migrations, creating the schema_migrations table (see “About the schema_migrations table” section below) if missing. It will also invoke the db:schema:dump command, which will update your db/schema.rb file to match the structure of your database.

To roll the database back to a previous migration version, use bin/rails db:rollback VERSION=X where X is the version to which you wish to downgrade. Alternatively, you can also use the STEP option if you wish to rollback last few migrations. bin/rails db:rollback STEP=2 will rollback the latest two migrations.

If any of the migrations throw an ActiveRecord::IrreversibleMigration exception, that step will fail and you'll have some manual work to do.

More examples

Not all migrations change the schema. Some just fix the data:

class RemoveEmptyTags < ActiveRecord::Migration[6.0]
  def up
    Tag.all.each { |tag| tag.destroy if tag.pages.empty? }
  end

  def down
    # not much we can do to restore deleted data
    raise ActiveRecord::IrreversibleMigration, "Can't recover the deleted tags"
  end
end

Others remove columns when they migrate up instead of down:

class RemoveUnnecessaryItemAttributes < ActiveRecord::Migration[6.0]
  def up
    remove_column :items, :incomplete_items_count
    remove_column :items, :completed_items_count
  end

  def down
    add_column :items, :incomplete_items_count
    add_column :items, :completed_items_count
  end
end

And sometimes you need to do something in SQL not abstracted directly by migrations:

class MakeJoinUnique < ActiveRecord::Migration[6.0]
  def up
    execute "ALTER TABLE `pages_linked_pages` ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
  end

  def down
    execute "ALTER TABLE `pages_linked_pages` DROP INDEX `page_id_linked_page_id`"
  end
end

Using a model after changing its table

Sometimes you'll want to add a column in a migration and populate it immediately after. In that case, you'll need to make a call to Base#reset_column_information in order to ensure that the model has the latest column data from after the new column was added. Example:

class AddPeopleSalary < ActiveRecord::Migration[6.0]
  def up
    add_column :people, :salary, :integer
    Person.reset_column_information
    Person.all.each do |p|
      p.update_attribute :salary, SalaryCalculator.compute(p)
    end
  end
end

Controlling verbosity

By default, migrations will describe the actions they are taking, writing them to the console as they happen, along with benchmarks describing how long each step took.

You can quiet them down by setting ActiveRecord::Migration.verbose = false.

You can also insert your own messages and benchmarks by using the say_with_time method:

def up
  ...
  say_with_time "Updating salaries..." do
    Person.all.each do |p|
      p.update_attribute :salary, SalaryCalculator.compute(p)
    end
  end
  ...
end

The phrase “Updating salaries…” would then be printed, along with the benchmark for the block when the block completes.

Timestamped Migrations

By default, Rails generates migrations that look like:

20080717013526_your_migration_name.rb

The prefix is a generation timestamp (in UTC).

If you'd prefer to use numeric prefixes, you can turn timestamped migrations off by setting:

config.active_record.timestamped_migrations = false

In application.rb.

Reversible Migrations

Reversible migrations are migrations that know how to go down for you. You simply supply the up logic, and the Migration system figures out how to execute the down commands for you.

To define a reversible migration, define the change method in your migration like this:

class TenderloveMigration < ActiveRecord::Migration[6.0]
  def change
    create_table(:horses) do |t|
      t.column :content, :text
      t.column :remind_at, :datetime
    end
  end
end

This migration will create the horses table for you on the way up, and automatically figure out how to drop the table on the way down.

Some commands cannot be reversed. If you care to define how to move up and down in these cases, you should define the up and down methods as before.

If a command cannot be reversed, an ActiveRecord::IrreversibleMigration exception will be raised when the migration is moving down.

For a list of commands that are reversible, please see ActiveRecord::Migration::CommandRecorder.

Transactional Migrations

If the database adapter supports DDL transactions, all migrations will automatically be wrapped in a transaction. There are queries that you can't execute inside a transaction though, and for these situations you can turn the automatic transactions off.

class ChangeEnum < ActiveRecord::Migration[6.0]
  disable_ddl_transaction!

  def up
    execute "ALTER TYPE model_size ADD VALUE 'new_value'"
  end
end

Remember that you can still open your own transactions, even if you are in a Migration with self.disable_ddl_transaction!.