app/models/moderatorship.rb
class Moderatorship < ApplicationRecord
belongs_to :user
belongs_to :forum
validates :user_id, :forum_id, presence: true
validate :uniqueness_of_relationship
attr_protected :forum_id, :user_id, :tenant_id
protected
def uniqueness_of_relationship
if self.class.exists?(:user_id => user_id, :forum_id => forum_id)
errors.add(:base, "Cannot add duplicate user/forum relation")
end
end
end