irb_session.rb

ruby-1.8.7-p249 > u1 = User.first(:username => "mtomer")
 => #<User username: "mtomer", _id: $oid4b4df1f6af64e72ac6000001, master_user_id: nil> 

ruby-1.8.7-p249 > u2 = User.first(:username => "mtomer2")
 => #<User username: "mtomer2", _id: $oid4b869089af64e75e58000003, master_user_id: $oid4b4df1f6af64e72ac6000001> 

ruby-1.8.7-p249 > u1._id.class
 => BSON::ObjectID 

ruby-1.8.7-p249 > u2.master_user_id.class
 => BSON::ObjectID 

ruby-1.8.7-p249 > u1._id == u2.master_user_id
 => true 

ruby-1.8.7-p249 > User.all(:master_user_id => u1._id)
 => []