bug in datamapper

gistfile1.rb

class Transaction
  include DataMapper::Resource

  property :user_id, Integer
  
  def self.u1
    all(:conditions => ["user_id != 4"])
  end
  def self.u2
    all(:conditions => ["user_id != 5"])
  end
  def self.u3
    all(:conditions => ["user_id != 6"])
  end
end

Console:

Transaction.u1.count
 SQL (0.270ms)  SELECT COUNT(*) FROM `transactions` WHERE (user_id != 4)

Transaction.u1.u2.count
 SQL (0.311ms)  SELECT COUNT(*) FROM `transactions` WHERE ((user_id != 4) AND (user_id != 5))

Transaction.u1.u2.u3.count
 SQL (0.312ms)  SELECT COUNT(*) FROM `transactions` WHERE ((user_id != 5) AND (user_id != 6))