title: Rails 3.2: Invoking up\down from another migration
body: <p>I'm looking to invoke another migration in a similar fashion to that of generators.
  Basically if you have a create table then at some point of time in the future you're no longer using the table and you want a migration to call <code>up</code> and <code>down</code> exactly opposite to those of the original create migration.
  If it's possible then I'd create a generator something like</p>
  
  <pre><code>rails g reverse_migration CreateModel
  </code></pre>
  
  <p>and then the result is something like</p>
  
  <pre><code>class ReverseCreateModel &lt; ActiveRecord::Migration
    def up
      #call to create model down
    end
    def down
      #call to create model up
    end
  end
  </code></pre>
  
  <p>I don't want any workaround kind of way and rather explicitly duplicate code and keep the ability for a clean migration and roleback.</p>
  
  <p>Any help would be greatly appreciated!</p>