Blacklist

blacklist.rb

module Blacklist
	extend ActiveSupport::Concern

	module ClassMethods
		def where_blacklist(query)
			where(query)
		end
	end
	
	def self.included(r)
		r.extend ClassMethods
	end
end

class Client < ActiveRecord::Base
  belongs_to :club
end

class Club < ActiveRecord::Base
  include Blacklist
  has_many :clients
end

Club.where_blacklist("clients.id NOT IN (1,2,3,4)")