Switch branches/tags
Nothing to show
Find file
Fetching contributors…
Cannot retrieve contributors at this time
76 lines (44 sloc) 1.88 KB


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)


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, options)

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

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 Methods

Foreigner adds extra methods to change_table.

Add a missing foreign key to comments:

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

Remove an unwanted foreign key:

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

Finding Missing Foreign Keys

Use Immigrant to generate a migration that includes all missing foreign keys.


Copyright © 2012 Matthew Higgins, released under the MIT license