Diamond multiple inheritance example in docs gives a clashing field check error

[Luc Saffre]

I need help with getting to test some code which is AFAICS equivalent to the second example (Piece, Book, Article, BookReview) of the ​Multiple inheritance section of the Models topic guide.

To reproduce my problem, please download the following files from ​this directory of my repository:

• ​main/models.py
• main/__init__.py
• manage.py
• settings.py

and then run:

$ python manage.py check

The output is:

SystemCheckError: System check identified some issues:

ERRORS:
main.PizzeriaBar: (models.E005) The field 'restaurant_ptr' from parent model 'main.bar' clashes with the field 'restaurant_ptr' from parent model 'main.pizzeria'.

The failure works in Django versions 1.11.2, 1.10.7, 1.9.13 (and maybe earlier as well).

I could not find any relevant difference between my code and the example in the documentation.

Any hints?

[Tim Graham]

It looks like the example in the documentation (added in #22442) never worked. I tested back to Django 1.7 and the check error is the same. If it cannot be fixed, perhaps it should be removed.

[Jeremy Satterfield]

The PizzeriaBar model is inheriting the restaurant_ptr field from both Pizzeria and Bar, so you need to specify the parent link field on at least one of them, per ​https://docs.djangoproject.com/en/dev/topics/db/models/#specifying-the-parent-link-field. This gets a little complicated if you are creating this child model and overriding these field names at a later date than the parent since you are changing the field name, it is going to require using the db_column arg on the field or more complex migrations.

This seems to me to have a lot of caveats to be provided as an example in the docs; however, here is a PR that updates the existing example to be working.

[Luc Saffre]

I confirm that this fixes the problem. Yes, diamond inheritance has more problems than this (see ​my blog for an example) and I currently avoid it in my projects. But still it is a feature for Django and I recommend to not remove if from the docs (but to rather write more about it).

[Tim Graham <timograham@…>]

In 473ab461:

Fixed #28332 -- Fixed diamond inheritence example in docs.

[Tim Graham <timograham@…>]

In e921e98:

[1.11.x] Fixed #28332 -- Fixed diamond inheritence example in docs.

Backport of 473ab4610ef90be05f09127aa37cd20bcda5875e from master