Support for foreign key constraint in Rails

Question

In Ruby on Rails, how to add foreign key constraint in migration?

Answer 1

AFAIK, there isn't any built-in support for that, but there are several plugins that help you with that. You can also add them manually to your migration files, just use the execute method for that, e.g. (sample from Rails API):

  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

Answer 2

http://blog.hasmanythrough.com/2007/1/15/basic-rails-association-cardinality

Answer 3

Here's a gem-based solution that includes support for adding and removing foreign key constraints, doesn't fail with sqlite, and works correctly with schema.rb files:

http://github.com/matthuhiggins/foreigner

Answer 4

This is an update to the matthuhiggins-foreigner gem: http://github.com/sparkfly/foreigner

Features:

• rspec coverage, tested against PostgreSQL 8.3.9 and MySQL 5.0.90
• Migration support
• schema.rb support

Future versions will include CHECK constraints for PostgreSQL, which is needed to implement multi-table inheritance.