How do I change an existing Comment model so it can belong to many different Models?

Question

In my rails app, Comment belongs_to Page (and stores a page_id), while Page has_many comments. But now I want Comment to belong to another model as well, say Picture. The Railscasts on Polymorphic associations discusses how to initially set up Commentable as the intermediary between Comment and the models it will belong_to.

However, I already have an existing Comment - Page setup. How do I migrate the database column and data over to a new Commentable setup so nothing breaks?

Answer 1

Try this:

Rename the column page_id to commentable_id and create an additional column called commentable_type and set it as default: 'page' so that the previous data in table is appropriate.

The migrations should be:

1) Migration for renaming page_id to commentable_id

class RenameColumnPageIdToCommentableIdInComments < ActiveRecord::Migration
  def up
    rename_column :comments, :page_id, :commentable_id       
  end

  def down
    rename_column :comments, :commentable_id, :page_id      
  end
end

2) Migration for adding commentable_type

class AddColumnCommentableTypeToComments< ActiveRecord::Migration
  def up        
    add_column :comments, :commentable_type, :string, default: 'Page'
  end

  def down
    rename_column :comments, :commentable_type,
  end
end

You will then need to alter the controller code for comments as well as the views. Hope that helps :)

Answer 2

Do you have content (actual pages and comments that you wish to preserve?

If not, just use the polymorphic associations, and it should work fine.

If you have data, back up the database, implement polymorphic associations, and then use a script to migrate the content from the old schema to the new.

If this is a large distributed app in production, then there are several other considerations.