<p>In Ruby on Rails, how to add foreign key constraint in migration?</p>

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
```
```