DictField is auto-dereferenced if storing data containing _cls property

[robertaistleitner]

This bug already existed some time ago (#1058) and there was even a bugfix for it (#1088) but I'm still experiencing this issue..

The bugfix doesn't work for me and I think the test case is not written in a correct way. I'm not sure but it looks like the DictField is auto-dereferenced if it's loaded from the database - which never happens in the test case.

I was able to track down the issue to it's root cause, which is here: https://github.com/MongoEngine/mongoengine/blob/master/mongoengine/base/document.py#L705

The _auto_dereference setting for the DictField is just ignored and overridden, which causes strange behaviour when accessing the field. First it looks like it's a normal instance of a Document, then it's a list with just the property names.

[robertaistleitner]

@thedrow would be awesome if you could join the discussion over here again.. 😉

[robertaistleitner]

@thedrow do you have any (good) news for me? stumbled again over this issue as this needs a workaround every time..

[robertaistleitner]

I just found another issue caused by this bug.. Saving the same document more than one time doesn't work because it replaces the DictField value with a DBRef, which causes validation to fail at the second save.

[bagerard]

The following is also a funky side effect of it:

class House(me.Document):
    address = me.StringField()
    meta = {'allow_inheritance': True}


class Person(me.Document):
    age = me.IntField()

Person(age=79).save()

# Let's force a house document in the person collection
dumped_house = House(address='garbage').to_mongo().to_dict()
Person._get_collection().insert_one(dumped_house)

print list(Person.objects())        # [<Person: Person object>, <House: House object>]