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This article tries to explain how to convert existing class in Ruby on Rails to a class that can be used in polymorphic association class. As the beginning, I do recommend to read article about polymorphic associations. It explains what they are and how they work.

The whole process will be described on classes Picture (represents picture) and Item (an item that needs a picture).

1. First of all, it is necessary to create columns for the pictures table. The best way is to use migrations. So, create file e.g. 013_convert_pictures_to_polymorph.rb in your db/migrate directory. It should contain something like:

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class ConvertPicturesToPolymorph < ActiveRecord::Migration
 
  def self.up
    # Change old item_id identifier to new picturable_id
    rename_column :pictures, :item_id, :picturable_id
    # Add type of association
    add_column :pictures, :picturable_type, :string, :length => 50
    # Fill in the type of association
    execute "update pictures set picturable_type = \"Item\""
  end
 
 
  def self.down
    # Bring it back
    rename_column :pictures, :picturable_id, :item_id
    remove_column :pictures, :picturable_type
  end
end

now run:

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rake migrate

to create the columns.

2. Once the appropriate columns were created, it is necessary to change the Item class. Change the

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  has_many  :pictures

to

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  has_many  :pictures, :as => :picturable

3. Now change the Picture class. Change

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  belongs_to :item

to

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  belongs_to :picturable, :polymorphic => true

Make sure, there is no “item” left in the Picture class. If so, change it to picturable and make sure all picturables have the method you are using.

4. Creating a new class enriched by pictures is simple. Just add

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  has_many  :pictures, :as => :picturable

to your class.

5. Last but not least, change your fixtures. The easiest way is to export the current data from your database. For this purpose I do use manage_fixtures plugin.

Good luck!