Weirdness with mongoengine ReferenceField

Question

This is a puzzling problem that's difficult even to name, let alone describe. I'll start with the essential facts and then give what background information might be relevant.

Consider two mongoengine document models:

class Bar(Document):
    # ...
    # field definitions
    # ...
    def bar_func(self):
        pass  # ...or some arbitrary code


class Foo(Document):
    bar = ReferenceField(Bar)

The following is inconsistently producing an AttributeError on our production server:

# Assume foo_id references a valid Foo document in Mongo
# and that its 'bar' reference is to a valid Bar document.
foo = Foo.objects.with_id(foo_id)
foo.bar.bar_func()  # <-- AttributeError on 'bar_func'

If I place debugging code right before the location of the error, evaluating type(foo.bar) as a string produces <class 'bson.dbref.DBRef'>. Obviously a DBRef doesn't have a bar_func attribute, but why is a DBRef being returned instead of an instance of Bar?

Further debugging code shows that the following condition is failing in the ReferenceField.__get__ function in mongoengine/fields.py:

if isinstance(value, (pymongo.dbref.DBRef)):
        value = _get_db().dereference(value)

But (pymongo.dbref.DBRef) is actually bson.dbref.DBRef, which seems to be the same as type(foo.bar)! Why does isinstance fail?

This is where things get really weird:

id(type(foo.bar)) == id(bson.dbref.DBRef)  # <-- Evaluates to False!

In other words, type(foo.bar) is a different bson.dbref.DBRef than the one obtained by referencing bson.dbref.DBRef directly. In fact, inspecting the __dict__ of these two types shows different memory locations for their functions and properties.

Note: I'll call the type returned by type(foo.bar) fooDBRef for convenience below, to distinguish it from the type referenced by bson.dbref.DBRef.

To debug further, I modified the DBRef code to add a metaclass that inspects the system modules at the time the DBRef type is created, and stores a list of the IDs of those modules in an extra class attribute of DBRef. The results show that the set of modules in existence when fooDBRef is created is entirely distinct from the set of modules in existence when the bare bson.dbref.DBRef type is created. All module IDs for one are different from all module IDs for the other.

Some possibly relevant factors:

• The server on which this error occurs runs mod_wsgi under Apache.
• The server runs two different Django sites under wsgi (call them site_a and site_b).
• Foo is defined in site_a.foo_app.models and Bar is defined in site_b.bar_app.models.
• site_a settings.py has site_b.bar_app in INSTALLED_APPS.
• The requests that produce the error are handled by site_a.
• There were site_b.* modules in sys.modules when fooDBRef was created, but no site_a.* modules. The reverse is true for bson.dbref.DBRef.
• After an httpd reload the error sometimes goes away for a little while, and returns sometime within 0-10 attempts.

Can anyone help me figure out what is causing fooDBRef to be different from bson.dbref.DBRef?