[ruby-on-rails] Rails 3 migrations: Adding reference column?

If I create a new rails 3 migration with (for example)

rails g migration tester title:tester user:references

, everything works fine...however if I add a column with something along the lines of:

rails g migration add_user_to_tester user:references

the reference field is not recognised. In short, the question is: how do I add a referencing column to a rails migration from the command line?

You can add references to your model through command line in the following manner:

rails g migration add_column_to_tester user_id:integer

This will generate a migration file like :

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

This works fine every time i use it..

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

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

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

That will do the trick:

rails g migration add_user_to_tester user_id:integer:index

When adding a column you need to make that column an integer and if possible stick with rails conventions. So for your case I am assuming you already have a Tester and User models, and testers and users tables.

To add the foreign key you need to create an integer column with the name user_id (convention):

add_column :tester, :user_id, :integer

Then add a belongs_to to the tester model:

class Tester < ActiveRecord::Base
  belongs_to :user
end

And you might also want to add an index for the foreign key (this is something the references already does for you):

add_index :tester, :user_id

EDIT: This is an outdated answer and should not be applied for Rails 4.x+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

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

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

For Rails 4

The generator accepts column type as references (also available as belongs_to).

This migration will create a user_id column and appropriate index:

$ rails g migration AddUserRefToProducts user:references 

generates:

class AddUserRefToProducts < ActiveRecord::Migration
  def change
    add_reference :products, :user, index: true
  end
end

http://guides.rubyonrails.org/active_record_migrations.html#creating-a-standalone-migration

For Rails 3

Helper is called references (also available as belongs_to).

This migration will create a category_id column of the appropriate type. Note that you pass the model name, not the column name. Active Record adds the _id for you.

change_table :products do |t|
  t.references :category
end

If you have polymorphic belongs_to associations then references will add both of the columns required:

change_table :products do |t|
  t.references :attachment, :polymorphic => {:default => 'Photo'}
end

Will add an attachment_id column and a string attachment_type column with a default value of Photo.

http://guides.rubyonrails.org/v3.2.21/migrations.html#creating-a-standalone-migration