Use lazy evaluated ForeignKey references to prevent future problems

[MarkusH]

Starting with Django 1.7 there have been multiple people hitting #django
with problems that eventually could be solved using lazy references to
related models instead of class references. Teaching it this ways seems
to be a good way to prevent future issues.

[MattBlack85]

I'm just curious about this issue, could you provide some more details about this problem? Also is still the same using settings.AUTH_USER_MODEL?

[MarkusH]

The problem, as far as I understand it, is that under some, relatively rare, conditions models are imported twice which screws up some internal references in Django, even though the new app loading in 1.7 should prevent that.

settings.AUTH_USER_MODEL already is a string: 'auth.User'. In order to not make the tutorial too complex at this point, I directly wrote the string representation and not a settings variable there.

Last but not least, the string representation in the form app_label.model_name works always, while the class references don't work if you refer to a model that is defined somewhere below:

class ModelA(models.Model):
    fk = models.ForeignKey('someapp.ModelB')

class ModelB(models.Model):
    fk = models.ForeignKey('someapp.ModelA')

The foreign key definition in ModelB could of course point to ModelA.

[olasitarska]

Thanks Markus! ❤️