"Unknown key: polymorphic" while running rake

Question

I want to implement a simple polymorphy. My goal is that user, group, and maybe other models can share a single pool of IDs, so that everything represents a GlobalObject. I built something like this:

• global_object.rb generated by $ rails generate scaffold GlobalObject mode:integer:

  class GlobalObject < ActiveRecord::Base
    attr_accessible :mode
    has_many :identifyable, :polymorphic => true, :dependent => :destroy
  end
  

• user.rb generated by $ rails generate scaffold User login password:

  class User < ActiveRecord::Base
    attr_accessible :login, :password, :salt
    belongs_to :global_object, :as => :permittable
  end
  

And a group model would follow with a very similar structure. I didn't modify the migrate files from the db folder, but just executed $ rake db:migrate.

• 20131102224115_create_global_objects.rb:

  class CreateGlobalObjects < ActiveRecord::Migration
    def change
      create_table :global_objects do |t|
        t.integer :mode
        t.timestamps
      end
    end
  end
  

• 20131102224332_create_users.rb:

  class CreateUsers < ActiveRecord::Migration
    def change
      create_table :users do |t|
        t.string :login
        t.string :password
        t.timestamps
      end
    end
  end
  

When I execute $ rake, it shows two impressive traces with these beginnings:

(...)hash/keys.rb:51:in `block in assert_valid_keys': Unknown key: polymorphic (ArgumentError)
    from /Users(...)
    ...

(...)hash/keys.rb:51:in `block in assert_valid_keys': Unknown key: polymorphic (ArgumentError)
    from /Users(...)
    ...

When I remove the :polymorphic => true in global_object.rb, the same errors occur, but instead of Unknown key: polymorphic, it says Unknown key: as. What is wrong?

Answer 1

There are several problems here:

• You are missing the polymorphic relationship columns in you migration that should refer to identifyable_id and identifyable_type.
• You are referring to the polymorphic association by different names - identifyable and permittable.
• Polymorphic relationships are meant to be set on the child elements (belongs_to) and referred to by multiple parent classes (has_many). You have it the other way around which is not supported (and wouldn't really make sense from a modeling perspective).

It would be helpful to understand what a GlobalObject is meant to accomplish to help determine if polymorphic associations are the correct approach here. But if you wanted to use them, you'd either need to make a GlobalObject belong_to identifyable (and add the necessary columns there), or you'd need to move it to the User and add the columns there.