Log: reproducing conflicting-mods issue in Mongoid using Mongo 2.4.2; modifications missing after reload

gistfile1.txt

Loading development environment (Rails 3.2.13)
2.0.0-p0 :001 > t = Train.create!
  MOPED: 127.0.0.1:27017 COMMAND      database=admin command={:ismaster=>1} (1.2758ms)
  MOPED: 127.0.0.1:27017 INSERT       database=mongoid_conflicting_mods_development collection=trains documents=[{"_id"=>"51745e04e3defeea70000001"}] flags=[]
                         COMMAND      database=mongoid_conflicting_mods_development command={:getlasterror=>1, :safe=>true} (25.0471ms)
 => #<Train _id: 51745e04e3defeea70000001, > 
2.0.0-p0 :002 > t.update_attributes(cars_attributes: [{capacity: 2}])
  MOPED: 127.0.0.1:27017 UPDATE       database=mongoid_conflicting_mods_development collection=trains selector={"_id"=>"51745e04e3defeea70000001"} update={"$set"=>{"seats"=>[{"_id"=>"51745e08e3defeea70000002"}, {"_id"=>"51745e08e3defeea70000003"}]}, "$pushAll"=>{""=>[{"_id"=>"51745e08e3defeea70000004", "seats"=>[{"_id"=>"51745e08e3defeea70000002"}, {"_id"=>"51745e08e3defeea70000003"}]}]}} flags=[]
                         COMMAND      database=mongoid_conflicting_mods_development command={:getlasterror=>1, :safe=>true} (0.9749ms)
 => true 
2.0.0-p0 :003 > t.cars
 => [#<Car _id: 51745e08e3defeea70000004, >] 
2.0.0-p0 :004 > t.cars.first.seats
 => [#<Seat _id: 51745e08e3defeea70000002, >, #<Seat _id: 51745e08e3defeea70000003, >] 
2.0.0-p0 :005 > t.reload
  MOPED: 127.0.0.1:27017 QUERY        database=mongoid_conflicting_mods_development collection=trains selector={:_id=>"51745e04e3defeea70000001"} flags=[] limit=-1 skip=0 batch_size=nil fields=nil (0.6881ms)
 => #<Train _id: 51745e04e3defeea70000001, : [{"_id"=>"51745e08e3defeea70000004", "seats"=>[{"_id"=>"51745e08e3defeea70000002"}, {"_id"=>"51745e08e3defeea70000003"}]}], seats: [{"_id"=>"51745e08e3defeea70000002"}, {"_id"=>"51745e08e3defeea70000003"}]> 
2.0.0-p0 :006 > t.cars
 => [] 
2.0.0-p0 :007 >