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Rails does not come with methods to add foreign keys. Foreigner introduces a few methods to your migrations for adding and removing foreign key constraints.

Since each adapter implements the API, migrations using Foreigner will continue to work on databases that do not support foreign keys, such as sqlite3.


In Rails 2, install as a gem by adding the following to config/environment.rb:

config.gem 'foreigner'

In Rails 3, install as a gem by adding the following to your Gemfile:

gem 'foreigner'


An adapter implementing the Foreigner API implements three methods. (Options are documented in connection_adapters/abstract/schema_definitions.rb):

add_foreign_key(from_table, to_table, options)
remove_foreign_key(from_table, options)


The most common use of foreign keys is to reference a table that a model belongs to. For example, given the following model:

class Comment < ActiveRecord::Base
  belongs_to :post

class Post < ActiveRecord::Base
  has_many :comments, :dependent => :delete_all

You should add a foreign key in your migration:

add_foreign_key(:comments, :posts)

The :dependent option can be moved from the has_many definition to the foreign key:

add_foreign_key(:comments, :posts, :dependent => :delete)

If the column is named article_id instead of post_id, use the :column option:

add_foreign_key(:comments, :posts, :column => 'article_id')

A name can be specified for the foreign key constraint:

add_foreign_key(:comments, :posts, :name => 'comment_article_foreign_key')

Change Table Shorthand

Foreigner adds extra behavior to change_table, which lets you define foreign keys using shorthand.

Add a missing foreign key to comments:

change_table :comments do |t|
  t.foreign_key :posts, :dependent => :delete

t.foreign_key accepts the same options as add_foreign_key.

Additional t.references option

Foreigner extends table.references with the :foreign_key option. Pass true, and the default foreign key options are used:

change_table :comments do |t|
  t.references :post, :foreign_key => true

An options hash can also be passed. It accepts the same options as add_foreign_key:

change_table :comments do |t|
  t.references :author, :foreign_key => {:dependent => :restrict}

By default, t.references will not generate a foreign key.


Similar to indexes, the foreign keys in your database are automatically dumped to schema.rb. This allows you to use foreign keys without switching to the :sql schema.

Copyright © 2009 Matthew Higgins, released under the MIT license