Method resolution order (MRO) error when Document is involved in diamond inheritance.

[alainbryden]

Trying to create a diamond inheritance structure using a class that inherits from a Mongo Document fails. The simplest example is just:

from mongoengine import Document
class A(Document):
    meta = {'allow_inheritance': True}
class B(A):
    pass
class C(A):
    pass
class D(B,C):
    pass

This results an error while constructing the Bat class: TypeError: Error when calling the metaclass bases Cannot create a consistent method resolution order (MRO) for bases DoesNotExist, DoesNotExist in metaclasses.pyc.

This more elaborate example should demonstrate what I wish to accomplish (and why a mixin approach wouldn't work). I intend to use each of these types in ReferenceFields, which require the classes to inherit Document.

from mongoengine import Document, fields

class Animal(Document):
    meta = {'allow_inheritance': True, 'collection': 'animals'}
    id = fields.UUIDField(primary_key=True, required=True)
    food = fields.StringField(required=True)
class Mammal(Animal):
    breathing_rate = fields.FloatField(required=True)
class WingedAnimal(Animal):
    wing_span = fields.FloatField(required=True)
class Bat(Mammal, WingedAnimal):
    pass

class Anthropologist(Document):
    favourite_mammal = fields.ReferenceField(Mammal, dbref=True)
    favourite_winged = fields.ReferenceField(WingedAnimal, dbref=True)