gitlab-org--gitlab-foss/app/models/subscription.rb

class Subscription < ActiveRecord::Base
  belongs_to :user
  belongs_to :project
  belongs_to :subscribable, polymorphic: true

  validates :user, :subscribable, presence: true

  validates :project_id, uniqueness: { scope: [:subscribable_id, :subscribable_type, :user_id] }
end