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
  def self.up
    add_column :accounts, :ssl_enabled, :boolean, :default => 1
  end

  def self.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 class 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
  def self.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 self.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

• 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.

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

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

• 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.

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

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

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

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

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

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

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.

You may then edit the self.up and self.down methods of MyNewMigration.

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

rails generate migration add_fieldname_to_tablename fieldname:string

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

class AddFieldnameToTablename < ActiveRecord::Migration
  def self.up
    add_column :tablenames, :fieldname, :string
  end

  def self.down
    remove_column :tablenames, :fieldname
  end
end

To run migrations against the currently configured database, use rake 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 task, 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 rake db:migrate VERSION=X where X is the version to which you wish to downgrade. If any of the migrations throw an ActiveRecord::IrreversibleMigration exception, that step will fail and you’ll have some manual work to do.

Database support

Migrations are currently supported in MySQL, PostgreSQL, SQLite, SQL Server, Sybase, and Oracle (all supported databases except DB2).

More examples

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

class RemoveEmptyTags < ActiveRecord::Migration
  def self.up
    Tag.find(:all).each { |tag| tag.destroy if tag.pages.empty? }
  end

  def self.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
  def self.up
    remove_column :items, :incomplete_items_count
    remove_column :items, :completed_items_count
  end

  def self.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
  def self.up
    execute "ALTER TABLE `pages_linked_pages` ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
  end

  def self.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
  def self.up
    add_column :people, :salary, :integer
    Person.reset_column_information
    Person.find(: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 self.up
  ...
  say_with_time "Updating salaries..." do
    Person.find(: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.

About the schema_migrations table

Rails versions 2.0 and prior used to create a table called schema_info when using migrations. This table contained the version of the schema as of the last applied migration.

Starting with Rails 2.1, the schema_info table is (automatically) replaced by the schema_migrations table, which contains the version numbers of all the migrations applied.

As a result, it is now possible to add migration files that are numbered lower than the current schema version: when migrating up, those never-applied “interleaved” migrations will be automatically applied, and when migrating down, never-applied “interleaved” migrations will be skipped.

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.

class ActiveRecord::ConnectionsAdapters::TableDefinition
def counter_caches(*args)
  args.each { |col| column("#{col}_count", :integer, :default => 0) }
end
end

class CreateUsers < ActiveRecord::Migration
def self.up
  create_table :users do |t|
    t.string :first_name, :last_name, :email
    t.counter_caches :photos, :messages
    t.timestamps
  end
end

def self.down
  drop_table :users
end
end

class ActiveRecord::Migration
def self.load_data(filename, dir = 'db/migrate/data')
  Fixtures.create_fixtures(File.join(RAILS_ROOT, dir), filename)
end
end

class CreateCountries < ActiveRecord::Migration
def self.up
  create_table :countries do |t|
    t.string :name, :code, :null => false
    t.timestamps
  end
  load_data :countries
end

def self.down
  drop_table :countries
end
end

  create_table :courses do |t|
    t.string :courseCode
  end

  create_table :courses_seasons, :id => false do |t|
    t.references :course, :null => false
    t.references :season, :null => false
  end
  add_index :courses_seasons, [:course_id, :season_id], :unique => true

end

def self.down
  drop_table :seasons
  drop_table :courses
  drop_table :courses_seasons
end
end

require 'create_ex_girl_friend_texts' # the migration file

class AddCurrentToVendor < ActiveRecord::Migration

class Vendor < ActiveRecord::Base
end

class Article < ActiveRecord::Base
  has_many :vendors, :class_name => 'AddCurrentToVendor::Vendor', :order => 'created_at'
end

def self.up
  add_column :vendors, :current, :boolean
  Article.all.each do |article|
    article.vendors.first.andand.update_attribute(:current, true)
  end
end

def self.down
  remove_column :vendors, :current
end
end

class AddLastNameToUsers < ActiveRecord::Migration
def self.up
  add_column :users, :last_name, :after => :first_name
end

def self.down
  remove_column :users, :last_name
end
end

class AddIdToUsers < ActiveRecord::Migration
def self.up
  add_column :urers, :id, :first => true
end

def self.down
  remove_column :users, :id
end
end

class AddUniquenessBarToFoo < ActiveRecord::Migration
class Foo < ActiveRecord::Base
end

def change

  add_column :foos, :bar, :string
  execute "ALTER TABLE foos ADD CONSTRAINT uk_foods_bar UNIQUE (bar)"    

  Foo.reset_column_information
  Foo.all.each do |f|
    begin
      #try get unique value in a new sub-transaction
      Foo.transaction(requires_new: true) do
        f.update_attributes!(:bar => "some ops. with another non-unique existent field to set this")
      end
    rescue ActiveRecord::StatementInvalid
       #We can't reuse a crashed transaction. New one.
       Foo.transaction(requires_new: true) do
        #Alternative unique value, if another error exist it's another
        #migration problem and then raise new error.
        f.update_attributes!(:bar => "some operation to set this-#{f.id}")
      end
    end
  end   
  change_column :foos, :bar, :string, :null => false

end
end