Check for duplicate model names

[Aymeric Augustin]

(Extracted from #21681.)

ModelBase.__new__ has an interesting behavior: it checks for a model class with the target name in the app registry and return it instead of creating a new class if found.

# myapp/models.py

from django.db import models

class MyModel(models.Model):
    pass

first_model_class = MyModel  # save a reference

class MyModel(models.Model):
    pass

second_model_class = MyModel  # save a reference

if second_model_class is first_model_class:
    raise RuntimeError("WTF")

It would be better to raise an error in that case than silently return the first model. See also #21679 which is essentially the same issue for applications.

[Aymeric Augustin]

This will likely be a problem for deferred queries.

[Anssi Kääriäinen]

We could cache the deferred classes in deferred_class_factory.

[Aymeric Augustin]

Yes yes yes. In fact I'd like to avoid storing these models in the app registry entirely.

[Aymeric Augustin]

This will also prevent importing twice the same models through different paths.

[Aymeric Augustin <aymeric.augustin@…>]

In f630373b929bc62bf4d66d60c532f7832e5fbe67:

Fixed #21711 -- Enforced unicity of model names.