no_dereference() disables dereferencing in other documents referencing document

[flacerdk]

As in this example:

import os

from mongoengine import Document, ReferenceField, connect


connect(host=os.getenv('DB'))


class A(Document):
    pass


class B(Document):
    a = ReferenceField(A)


class C(Document):
    b = ReferenceField(B)


a = A().save()
b = B(a=a).save()
c = C(b=b).save()

c = C.objects.get(id=c.id)

B.objects.no_dereference().get(id=c.b.id)
print(isinstance(c.b.a, A))  # prints False

B.objects.get(id=c.b.id)
print(isinstance(c.b.a, A))  # prints True

Is this expected behavior? Outside of the context manager, the documentation says nothing about no_dereference() affecting other querysets.

[flacerdk]

The no_dereference() example above works as I expect if I query using b.id instead:

c = C.objects.get(id=c.id)
B.objects.no_dereference().get(id=b.id)
print(isinstance(c.b.a, A))  # prints True

It looks like mongoengine is performing some kind of optimization and incidentally changing the object I'm using to query.

[bagerard]

Good catch! I confirm this is a bug, the .no_dereference() ends up setting B.a._auto_dereference=False until another B.objects() queryset gets used which resets B.a._auto_dereference to True.
Long story short, no_dereference() modifies some class attributes that is used when calling c.b.a
See below:

class A(Document):
    pass

class B(Document):
    a = ReferenceField(A)

_a = A().save()
_b = B(a=_a).save()

print B.a._auto_dereference    # True (default value)
B.objects.no_dereference().first()
print B.a._auto_dereference    # False <-- class attribute got modified
print id(B.a) == id(b._fields['a'])    # True, the A Field instance referenced in b points to the class attribute Field 
print b._fields['a']._auto_dereference    # False (since it is the class attribute)
print(isinstance(b.a, A))  # False (a remains a DBRef and does not get dereferenced)
B.objects().first()
print b._fields['a']._auto_dereference    # True, it gets reset'd
print(isinstance(b.a, A))  # True

I'll look into it in the next days.