app/models/remote_repo_site_user.rb
class RemoteRepoSiteUser < ActiveRecord::Base
belongs_to :site, class_name: 'RemoteRepoSite', foreign_key: 'remote_repo_site_id'
belongs_to :user
validates :site, presence: true
validates :email, presence: true, uniqueness: { scope: :remote_repo_site_id }
end