`id` alias overrides db_field resulting in inaccessible data

Fields cannot use db_field='id'. When the resulting data is accessed, the found object is clobbered by the primary key.

Here I set up a simple example where I attempt to store my_id in mongo under the id field name.

from mongoengine import *
connect('test_db') 
class MyObject(Document): 
    my_id = StringField(db_field='id')

obj = MyObject(my_id='obj_1')
obj.save()

It appears to work here as expected:

>>> print obj.id 

553fe1afe138235f8f252c92

>>> print obj.my_id

obj_1

It is also correctly stored in mongo.

>>> db.my_object.find()
{ "_id" : ObjectId("553fe1afe138235f8f252c92"), "id" : "obj_1" }

But the data cannot be retrieved using Mongoengine:

>>> found = MyObject.objects.first()
>>> print found.id

None

>>> print found.my_id

553fe1afe138235f8f252c92

>>> print found.to_json()

{"id": {"$oid": "553fe96be13823608b528766"}}

Start with the MyObject example and reproduce the mismatch through MyObject.objects.first(), found.id, found.my_id, and found.to_json(). Trace how the db_field='id' mapping is loaded and serialized, then verify that a saved object round-trips without clobbering either field and that the serialized data remains accessible.