The id type of full object and the id type of the lazy object are different

• Models:

import datetime
import uuid
from mongoengine import *

class Model1(Document):
    id = UUIDField(primary_key=True, default=uuid.uuid4, binary=False)
    attr1 = StringField(required=True)
    attr2 = IntField()
    attr3 = BooleanField()
    creation_date = DateTimeField(default=datetime.datetime.utcnow)
    meta = {
        'db_alias': 'testdb',
        'collection': 'model1'
    }


class Model2(Document):
    id = UUIDField(primary_key=True, default=uuid.uuid4, binary=False)
    attr1 = StringField()
    reference_field = LazyReferenceField('Model1', dbref=True)
    creation_date = DateTimeField(default=datetime.datetime.utcnow)
    meta = {
        'db_alias': 'testdb',
        'collection': 'model2'
    }

• Insert sample data (NOTE: the Earth object reference the Moon object):

moon = Model1()
moon.attr1 = 'Moon'
moon.attr2 = 22
moon.attr3 = False
moon.save()

earth = Model2()
earth.attr1 = 'Earth'
earth.reference_field = moon
earth.save()

• If I check that the Earth object references the moon object by comparing ids, the result is that it does not

earth = Model2.objects.get(attr1='Earth')
moon = Model1.objects.get(attr1='Moon')
if earth.reference_field.id == moon.id:
    print('The Earth reference to the Moon')
else:
    print('The Earth does NOT reference to the Moon')
# OUTPUT: The Earth does NOT reference to the Moon    <-- ¿WTF?

• The cause is that the id type of full object is UUID and the id type of the lazy object is string:

print('Types ids:', type(moon.id), type(earth.reference_field.id))
# OUTPUT: Types ids: <class 'uuid.UUID'> <class 'str'>    <-- the ids have different types!!!!

I can't use LazyReference objects if I can't easily compare ids without doing conversions.

Reproduce the mismatch with the Model1 and Model2 definitions and sample queries shown in the issue. Start by tracing how LazyReferenceField loads its id versus how UUIDField returns the full object's id. Done means the lazy and full references expose compatible UUID ids and a regression test covers the comparison.