Diamond inheritance¶

The lino_book.projects.diamond project was used in ancient times (before Django 1.11) to test a workaround for some problems with diamond inheritance.

Here is an example of diamond inheritance:

digraph foo {
Addressable -> Restaurant
Restaurant -> Bar
Restaurant -> Pizzeria
Pizzeria -> PizzaBar
Bar -> PizzaBar
}

from django.db import models


class Addressable(models.Model):

    class Meta:
        abstract = True

    street = models.CharField(max_length=255)


class Restaurant(Addressable):
    name = models.CharField(max_length=255)


class Bar(Restaurant):
    bar_restaurant = models.OneToOneField(Restaurant,
                                          parent_link=True,
                                          on_delete=models.CASCADE)
    min_age = models.IntegerField()


class Pizzeria(Restaurant):
    pizzeria_restaurant = models.OneToOneField(Restaurant,
                                               parent_link=True,
                                               on_delete=models.CASCADE)
    specialty = models.CharField(max_length=255)


class PizzaBar(Bar, Pizzeria):
    pizza_bar_specific_field = models.CharField(max_length=255)

The problem¶

>>> from lino import startup
>>> startup('lino_book.projects.diamond.settings')
>>> from lino.api.doctest import *

>>> p = main.PizzaBar(name="A", min_age="B", specialty="C",
...     pizza_bar_specific_field="Doodle", street="E")

Despite the fact that we specify a non-blank value for name, we had a database object whose name is blank, while the pizza_bar_specific_field field is not:

>>> print(p.name)
A
>>> print(p.pizza_bar_specific_field)
Doodle
>>> print(p.street)
E

Some Django versions raises a django.core.exceptions.FieldError saying that “Local field u’street’ in class ‘PizzeriaBar’ clashes with field of the same name from base class ‘Pizzeria’”.

The street field is defined in a parent of the common parent. Django then got messed up when testing for duplicate fields and incorrectly thinks that street is duplicated.

See Django ticket Django ticket #10808.