title: Rails migrations - change_column with type conversion
body: <p>I already google'd aroung a little bit and seems there's no satisfying answer for my problem.</p>
  
  <p>I have a table with column of type string.
  I'd like to run following migration:</p>
  
  <pre><code>class ChangeColumnToBoolean &lt; ActiveRecord::Migration
      def up
          change_column :users, :smoking, :boolean
      end
  end
  </code></pre>
  
  <p>When I run this I get following error</p>
  
  <pre><code>PG::Error: ERROR:  column "smoking" cannot be cast automatically to type boolean
  HINT:  Specify a USING expression to perform the conversion.
  : ALTER TABLE "users" ALTER COLUMN "smoking" TYPE boolean
  </code></pre>
  
  <p>I know I can perform this migration using pure SQL but still it would be nicer if I could do it with Rails. I went through Rails code and seems theres no such possibility, but maybe someone knows a way?</p>
  
  <p>I'm not interested in:
  - pure SQL
  - dropping the column
  - creating another column, converting data, dropping original and then renaming</p>