Has Many :through polymorphic problem

Admin.rb

class Admin < ActiveRecord::Base
  has_many :favorites, as: :favoritable
  has_many :favorite_services, through: 'favorites', source: 'service'
end

Favorite.rb

class Favorite < ActiveRecord::Base
  belongs_to :service
  belongs_to :favoritable, polymorphic: true
end

Service.rb

class Service < ActiveRecord::Base
  has_many :favorites
  has_many :admins_who_favorited, through: 'favorites', class_name: 'Admin', source: 'favoritable', source_type: 'Admin'
  has_many :users_who_favorited, through: 'favorites', class_name: 'User', source: 'favoritable', source_type: 'User'
end

User.rb

class User < ActiveRecord::Base
  has_many :favorites, as: :favoritable
  has_many :favorite_services, through: 'favorites', source: 'service'
end

Gist updated with problem solution.