title: Ruby on Rails - reverse add_column, add_index migration
body: <p>I have associations set up in survey model as follows</p>
  
  <pre><code>attr_accessible :name, :questions_attributes
  has_many :questions
  </code></pre>
  
  <p>and in question model</p>
  
  <pre><code>attr_accessible :content, :survey_id, :answers_attributes
  belongs_to :survey
  </code></pre>
  
  <p>I have a migration that adds question_id to survey table
  but I came to realise that this migration wouldn't be necessary as
  I have already defined their associations in the model.
  Is my understanding correct?</p>
  
  <p>If my understanding is correct, I would like to reverse that migration.</p>
  
  <p>I have added the last 2 lines starting with <code>remove</code> and ran <code>rake db:migrate</code>
  but it doesn't do anything.</p>
  
  <pre><code>def change
      add_column :surveys, :question_id, :integer
      add_index :surveys, :question_id
      remove_column :surveys, :question_id, :integer
      remove_index :surveys, :question_id
  end
  </code></pre>