No validation error when ForeignKey related_name clashes with manager name.

[Russell Keith-Magee]

Consider the following model definition:

class Author(models.Model):
    authors = models.Manager()

    name = models.CharField(max_length=100)

    mentor = models.ForeignKey('self', related_name='authors')

The related name on the the foreign key clashes with the manager, so when you try to issue a query:

    Author.authors.filter(name__startswith='Douglas')

you get an error because "Author.authors has no attribute 'filter'".

The problem is order dependent; if you define the manager *after* the foreign key, you get different errors.

I haven't checked what errors you get if you just have the default Manager, and a related_name of 'objects'.

There may also be problems if you have a field named 'objects'.

I suspect this class of problem could be picked up by the system check framework.

[Tim Graham]

As Loic noted on the PR, we should revisit this after the _meta factor is merged.

[Davide Ceretti]

Given loic's comment at ​https://github.com/django/django/pull/3359#issuecomment-59642918, what would be the correct implementation of this validation check? Adding it to BaseManager.check method?