Rails Migration Change vs Up & Down methods

Question

I am working through the book Rails Test Prescriptions and during the setup it is asking me to change a migration file to the following:

class ProjectUserJoin < ActiveRecord::Migration
  def self.up
    create_table :projects_users, :force => true, :id => false do |t|
      t.references :project
      t.references :user
      t.timestamps
    end
  end

  def self.down
    drop_table :projects_users
  end
end

It seems I am using a later version on Rails (4.0.0) than the book (2 or 3.x) and my migration file looks like this:

class ProjectUserJoin < ActiveRecord::Migration
  def change
  end
end

How do I edit the change method to do the same as the up and down methods above? So far I have tried using up and down as opposed to self.up and self.down and copying in the same code. It did not work.

Thanks!

Answer 1

Just change def change with def self.up content.

You can check the result by running rake db:migrate at your console - it will create the table (self.up functionality) and rake db:rollback - it will drop the table (self.down functionality).

Answer 2

Your up/down migration would look like this for change:

class ProjectUserJoin < ActiveRecord::Migration
  def change
    create_table :projects_users, :force => true, :id => false do |t|
      t.references :project
      t.references :user
      t.timestamps
    end
  end
end

The change method is able to automatically figure out the down actions required based upon the create/update information you provide. It is not a complete replacement for the original self.up/self.down methods however as some database actions you take Rails is not able to automatically figure out what is the corresponding up/down actions. On example of this is if you need to run an arbitrary SQL statement execute-<<SQL ... SQL.