Question

I’d like to know the “proper” way to approach adding a relation between two existing classes in Rails 3.

Given existing models: Clown & Rabbit

I’d like to add a reference (belongs_to) from Rabbit to Clown. I start by trying to generate a migration:

rails g migration AddClownToRabbits clown:reference

which gives me a migration that looks like:

class AddClownToRabbits < ActiveRecord::Migration
  def self.up
    add_column :rabbits, :clown, :reference
  end

  def self.down
    remove_column :rabbits, :clown
  end
end

After rake db:migrate on this migration I examine SQLite3’s development.db and see a new column: "clown" reference

I guess I was expecting a "clown_id" integer column and a migration that looked like:

class AddClownToRabbits < ActiveRecord::Migration
  def self.up
    add_column :rabbits, :clown_id
  end

  def self.down
    remove_column :rabbits, :clown_id
  end
end

I’m sure :reference is supposed to be equivalent to “t.references :clown” but I can’t find the documentation (big surprise). API says add_column: Instantiates a new column for the table. The type parameter is normally one of the migrations native types, which is one of the following: :primary_key, :string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, :boolean.

…with no reference to :reference.

Answer 1

After you set belongs_to in Rabbit, and has_many in Clown, you can do a migration with:

add_column :rabbit, :clown_id, :integer

EDIT: See Paulo’s answer below for a more updated answer (Rails 4+)