Rails 3 migrations: Adding reference column?

Question

If you are using the Rails 4.x you can now generate migrations with references, like this:

rails generate migration AddUserRefToProducts user:references

like you can see on rails guides

EDIT: This is an outdated answer and should not be applied for Rails 4.x+

You don't need to add references when you can use an integer id to your referenced class.

I'd say the advantage of using references instead of a plain integer is that the model will be predefined with belongs_to and since the model is already created and will not be affected when you migrate something existing, the purpose is kind of lost.

So I would do like this instead:

rails g migration add_user_id_to_tester user_id:integer

And then manually add belongs_to :user in the Tester model

Please note that you will most likely need an index on that column too.

class AddUserReferenceToTester < ActiveRecord::Migration
  def change
    add_column :testers, :user_id, :integer
    add_index  :testers, :user_id
  end
end

With the two previous steps stated above, you're still missing the foreign key constraint. This should work:

  class AddUserReferenceToTester < ActiveRecord::Migration
      def change
          add_column :testers, :user_id, :integer, references: :users
      end
  end

You can use references in a change migration. This is valid Rails 3.2.13 code:

class AddUserToTester < ActiveRecord::Migration
  def change
    change_table :testers do |t|
      t.references :user, index: true 
    end
  end
  def down
    change_table :testers do |t|
      t.remove :user_id
    end
  end
end

c.f.: http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_table

Running rails g migration AddUserRefToSponsors user:references will generate the following migration:

def change
  add_reference :sponsors, :user, index: true
end