Awesome

Foreigner

Foreigner introduces a few methods to your migrations for adding and removing foreign key constraints. It also dumps foreign keys to schema.rb.

The following adapters are supported:

mysql2
postgres
sqlite (foreign key methods are a no-op)

Foreigner was rendered obsolete in Rails 4.2. The migration DSL supports foreign keys out of the box via add_foreign_key and remove_foreign_key.

Installation

Add the following to your Gemfile:

gem 'foreigner'

API Examples

Foreigner adds two methods to migrations.

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

(Options are documented in connection_adapters/abstract/schema_statements.rb):

For example, given the following model:

class Comment < ActiveRecord::Base
  belongs_to :post
end

class Post < ActiveRecord::Base
  has_many :comments, dependent: :delete_all
end

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')

The :column and :name options create a foreign key with a custom name. In order to remove it you need to specify :name:

remove_foreign_key(:comments, name: 'comment_article_foreign_key')

Change Table Methods

Foreigner adds extra methods to create_table and change_table.

Create a new table with a foreign key:

create_table :products do |t|
  t.string :name
  t.integer :factory_id
  t.foreign_key :factories
end

Add a missing foreign key to comments:

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

Remove an unwanted foreign key:

change_table :comments do |t|
  t.remove_foreign_key :users
end

Database-specific options

Database-specific options will never be supported by foreigner. You can add them using :options:

add_foreign_key(:comments, :posts, options: 'ON UPDATE DEFERRED')

Foreigner Add-ons

immigrant - generate a migration that includes all missing foreign keys.
sqlserver-foreigner - A plugin for SQL Server.