to_json does not honor primary_key field

When using MongoEngine to abstract mongo (which is its purpose, I suppose), and when using one of my fields as the primary_key, to_json still returns the _id field. For example, in the below code, I would expect '_id' to be returned to me as 'hostname'.

class Device(Document):
    hostname=StringField(primary_key=True)
    cpu_load_average_15min=IntField()
    last_communication=FloatField()

for device in Device.objects.all():
    son.append(json.loads(device.to_json()))

# son is {"_id": "host3", "last_communication": 1389059449.072472}, {"_id": "host1", "cpu_load_average_15min": 0, "last_communication": 1389380999.455526}]

Obviously I can work around this, but in my opinion the point of an abstaction layer is to abstract away things like '_id'.

Would a pull request be welcome to fix this? I would anticipate adding a keyword argument honor_field_names that, when true, would account changing the _id fieldname. That argument would be False by default, To avoid backwards compatibility problems.

Start at the to_json entry point and reproduce the documented Device example with hostname as the primary_key. Determine how serialization currently represents _id and verify that the completed behavior returns the configured field name without breaking existing output expectations.