ដក​ស្រង់​ពី​ឯកសារ​ផ្លូវការ​របស់​មូលនិធិ Django

The second type of model inheritance supported by Django is when each model in the hierarchy is a model all by itself. Each model corresponds to its own database table and can be queried and created individually. The inheritance relationship introduces links between the child model and each of its parents (via an automatically-created OneToOneField). For example:

from django.db import models

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(Place):
    serves_hot_dogs = models.BooleanField(default=False)
    serves_pizza = models.BooleanField(default=False)

All of the fields of Place will also be available in Restaurant, although the data will reside in a different database table. So these are both possible:

>>> Place.objects.filter(name="Bob's Cafe")
>>> Restaurant.objects.filter(name="Bob's Cafe")

If you have a Place that is also a Restaurant, you can get from the Place object to the Restaurant object by using the lowercase version of the model name:

>>> p = Place.objects.get(id=12)
# If p is a Restaurant object, this will give the child class:
>>> p.restaurant
<Restaurant: ...>

However, if p in the above example was not a Restaurant (it had been created directly as a Place object or was the parent of some other class), referring to p.restaurant would raise a Restaurant.DoesNotExist exception.

place_ptr = models.OneToOneField(
    Place, on_delete=models.CASCADE,
    parent_link=True,
    primary_key=True,
)

You can override that field by declaring your own OneToOneField with parent_link=True on Restaurant.

In the multi-table inheritance situation, it doesn’t make sense for a child class to inherit from its parent’s Meta class. All the Meta options have already been applied to the parent class and applying them again would normally only lead to contradictory behavior (this is in contrast with the abstract base class case, where the base class doesn’t exist in its own right).

So a child model does not have access to its parent’s Meta class. However, there are a few limited cases where the child inherits behavior from the parent: if the child does not specify an ordering attribute or a get_latest_by attribute, it will inherit these from its parent.

If the parent has an ordering and you don’t want the child to have any natural ordering, you can explicitly disable it:

class ChildModel(ParentModel):
    # ...
    class Meta:
        # Remove parent's ordering effect
        ordering = []

Because multi-table inheritance uses an implicit OneToOneField to link the child and the parent, it’s possible to move from the parent down to the child, as in the above example. However, this uses up the name that is the default related_name value for ForeignKey and ManyToManyField relations. If you are putting those types of relations on a subclass of the parent model, you must specify the related_name attribute on each such field. If you forget, Django will raise a validation error.

For example, using the above Place class again, let’s create another subclass with a ManyToManyField:

class Supplier(Place):
    customers = models.ManyToManyField(Place)

This results in the error:

Reverse query name for 'Supplier.customers' clashes with reverse query
name for 'Supplier.place_ptr'.

HINT: Add or change a related_name argument to the definition for
'Supplier.customers' or 'Supplier.place_ptr'.

Adding related_name to the customers field as follows would resolve the error: models.ManyToManyField(Place, related_name='provider').

As mentioned, Django will automatically create a OneToOneField linking your child class back to any non-abstract parent models. If you want to control the name of the attribute linking back to the parent, you can create your own OneToOneField and set parent_link=True to indicate that your field is the link back to the parent class.