Examples using foreigner and foreigner-matcher

foreign_key_migration.rb

class ForeignKeyMigration < ActiveRecord::Migration
  def up
    add_foreign_key :user_logins, :users, :dependent => :delete
  end

  def down
    drop_foreign_key :user_logins, :users
  end
end

small_primer.txt

[1] http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html

* Referential Action Options*

The primary reference documentation [1] shows us three ways we can use foreign keys:

Delete child records upon deletion of a parent record (CASCADE)
Set foreign keys to NULL when deleting a parent record (SET NULL)
Prevent parent record deletion with a foreign key in child records (RESTRICTED, NO ACTION)

Notes...

The CASCADE option can be mimicked in Rails, giving us the option of managing deletion 
on database level or the application level.


* Option Examples *

These are meant to be examples and descriptions of what would/could happen should we use
the given option on existing data.

CASCADE

    children.parent_id -> parent.id

    If we delete a given parent record, it will in turn delete any children tied to it.

SET NULL

    children.parent_id -> parent.id

    If we delete a given parent record, it will in turn set child parent_id values to NULL.
    This would allow us to keep the children record intact and remove an orphan reference.

RESTRICTED, NO ACTION

    children.parent_id -> parent.id

    We have the ability do delete a given children record, but we cannot delete a parent record
    that has children tied to it. This would force us to delete all child records before we delete
    the parent record. On the application level, this can be accomplished via cascade deletion.

user.rb

class User < ActiveRecord::Base
  has_many :user_logins
end

user_login.rb

class UserLogin < ActiveRecord::Base
  belongs_to :user
end

user_login_spec.rb

require 'spec_helper'

describe UserLogin do
  # Mysql foreign keys require an index on the column that the constraint is applied to.
  # Foreigner will create it if it's not there, however a good habit is to do it yourself.
  # It's always better to know as much about the schema as possible!
  #
  # Note the attributes and indexes contexts use the format of remarkable.

  context 'attributes' do
    it { should have_column(:user_id, :type => :integer) }
  end

  context 'indexes' do
    it { should have_index(:user_id) }
  end

  context 'foreign keys' do
    it { should have_foreign_key_for(:user, :dependent => :delete) }
  end
end