TIL - add scope for has_many relationships by the model's specific attribute(s); not just by the standard rails model_id attribute

scope_for_has_many_relationships.rb

# add scope (with one or more attributes) for has_many relationships

# This is great for polyphomism when you want different models to be able to have subscriptions; so that you don't just attach only one model to subscriptions

# Reference: http://edgeguides.rubyonrails.org/association_basics.html#scopes-for-has-many

class Question < ActiveRecord::Base
  has_many :subscriptions, -> { where "type = 'Question'" }, :foreign_key => "subscribed_item_id"
end

class Subscription < ActiveRecord::Base
  # attributes: type, subscribed_item_id
end