mongoengine ListField(ReferenceField()) and custom primary_key

I have some pretty simple code: ``` from mongoengine import * class Comment(Document): id = IntField(primary_key=True) text = StringField() class Message(Document): id = IntField(primary_key=True) comments = ListField(ReferenceField(Comment)) connect('test_db') c1 = Comment(id=1) c1.text = 'message_one' c1.save() c2 = Comment(id=2) c2.text = 'message_two' c2.save() m = Message(id=1) m.comments = [c1, c2] m.save() msg = Message.objects.get(id=1) for comment in msg.comments: print comment.id, comment.text ``` I've expected that it will print 1 message_one 2 message_two but I've got 1 message_one 1 message_one When I use any mongodb admin UI to view database everything seems to be ok: { "_cls" : "Message" , "_id" : 1 , "_types" : [ "Message"] , "comments" : [ { "$ref" : "comment" , "$id" : 1} , { "$ref" : "comment" , "$id" : 2}]} I've tried to swap c1 and c2 in the code (for example use m.comments = [c2, c1]) and unexpectedly I've got the proper output: 2 message_two 1 message_one Also I've tried to not use custom primary key "id" and in both cases everything worked properly. I am very confused with this bug, it seems it's something wrong with mongoengine or I'm not using it in a right way. Please, any ideas?