Double inheritance causes metaclass conflict

Question

I use two django packages - django-mptt (utilities for implementing Modified Preorder Tree Traversal) and django-hvad (model translation).

I have a model class MenuItem and I want to it extends TranslatableModel and MPTTModel, like this:

class MenuItem(TranslatableModel, MPTTModel):

but it causes metaclass conflict:

(TypeError: Error when calling the metaclass bases 
metaclass conflict: the metaclass of a derived class 
must be a (non-strict) subclass of the metaclasses of all its bases)

What is the solution this problem? I hope that I can use double inheritance.

Why do you need multiple inheritance? It's usually best to avoid it when you can. — jathanism, Jun 29 '12 at 00:13
Because MenuItem has translations (need extends TranslatableModel) and must have tree hierarchy (need extends MPTTModel). — David Silva, Jun 29 '12 at 00:18
MPTTModel has metaclass MPTTModelBase and TranslatableModel has metaclass TranslatableModelBase. Just like the answer in the question that @jathanism linked to, you have to set your metaclass because Python doesn't know which base class' metaclass to use. — Jeremy Pridemore, Jun 29 '12 at 00:34
possible duplicate of [Triple inheritance causes metaclass conflict... Sometimes](http://stackoverflow.com/questions/6557407/triple-inheritance-causes-metaclass-conflict-sometimes) — JMax, Jul 25 '12 at 08:01

schacki · Answer 1 · 2012-08-15T08:48:56.550

5

You might want to do the following:

class CombinedMeta(TranslatableModel.__metaclass__, MPTTModel.__metaclass__):
    pass
class MenuItem(TranslatableModel, MPTTModel):
    __metaclass__=CombinedMeta

This should give you exactly what you want, without any error mesages.

edited Aug 15 '12 at 08:48

answered Aug 13 '12 at 19:43

schacki

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In this case then `MenuItem.objects` will be the translation manager. In order to add tree manager, it has to be added manually - `tobjects = TreeManager()` – miki725 Mar 25 '13 at 03:49

score 2 · Answer 2 · answered Dec 18 '12 at 08:20

2

Sorry for the late answer, but it i think it'll help for people who have same problem. When you subclass MPTTModel and another class, make sure you put MPTTModel first, like this:

class MenuItem(MPTTModel, TranslatableModel):

answered Dec 18 '12 at 08:20

Otskimanot Sqilal

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This is [what the docs recommend](https://django-mptt.github.io/django-mptt/models.html#setting-up-a-django-model-for-mptt) – Hamish Downer Nov 04 '16 at 10:31

score 1 · Answer 3 · answered Sep 11 '13 at 08:06

Generally the answer of @schacki would work. However, django-hvad overrides many other manager/queryset classes under the hood, which makes integrating with django-mptt/django-polymorphic and friends impossible at the moment.

Take a look at django-parler, which features a similar API and admin integration as django-hvad, but plays nice with other packages too. The table layout is identical, so you should be able to switch the packages easily.

Double inheritance causes metaclass conflict

3 Answers3

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