Question

Is it possible to implement, in Rails 3, an association

class ApiCredentials < ActiveRecord::Base
  belongs_to Model

end

where Model can be any model.

Then in the api_credentials table, I have fields like model_id and model to track this association.

I don’t assume that the convenient standard ActiveRecord queries will still work, but has anyone implemented something like this efficiently or used another method?

In the end, all I want to do is still to check if a particular model has ApiCredentials but I cannot know in advance whether the model will implement it.

Thanks in advance.

Answer 1

Sounds like you’re talking about a polymorphic association:

With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model.
[…]
You can think of a polymorphic belongs_to declaration as setting up an interface that any other model can use.

This even uses your model_id/model columns but calls them X_id and X_type where X is the association name:

class CreatePictures < ActiveRecord::Migration
  def change
    create_table :pictures do |t|
      #...
      t.integer :imageable_id
      t.string  :imageable_type
      #...
    end
  end
end

In your case, you’d have something like this:

class ApiCredential < ActiveRecord::Base
  belongs_to :creditable, :polymorphic => true
end

class Pancake < ActiveRecord::Base
  has_many :api_credentials, :as => :creditable
end

class OneCentStamp < ActiveRecord::Base
  has_many :api_credentials, :as => :creditable
end