Creating foreign key constraints in ActiveRecord

Question

How do I create foreign keys in ActiveRecord? I have something like the following in my models:

class Student < ActiveRecord::Base
  attr_accessible :name, :level_id
  belongs_to :level
end

class Level < ActiveRecord::Base
  attr_accessible :number
  has_many :students
end

But the schema.rb and development sqlite3 database don't have any indication foreign key constraints were setup with the level_id field. Is this something I have to do manually apart from ActiveRecord or Rails? Did I miss a step?

Using Rails 3.2.8

Answer 1

You do not need a foreign key constraints for ActiveRecord to correctly map the relationships. You can use validations to have the Rails app ensure data integrity.

Rails migration do not provider helpers to create foreign keys. You can create your own SQL for the constraint in the migration or use the the Foreigner Gem. Foreigner will provide helper methods for creating constraints in a migration:

add_foreign_key(:students, :levels)

Answer 2

If you have rails >= 4.2 and using the mysql, mysql2, or postgresql adapter, then you can use the add_foreign_key method in your migration like this:

# add a foreign key to `articles.author_id` referencing `authors.id`
add_foreign_key :articles, :authors

API Reference