Model Inheritance in Django
Models inheritance works the same way as normal Python class inheritance works, the only difference is, whether we want the parent models to have their own table in the database or not. When the parent model tables are not created as tables it just acts as a container for common fields and methods.
There are three styles of inheritance possible in Django.
- Abstract base classes : Use this when the parent class contains common fields and the parent class table is not desirable.
- Multi-table inheritance : Use this when the parent class has common fields, but the parent class table also exists in the database all by itself.
- Proxy models : Use this when you want to modify the behavior of the parent class, like by changing
orderbyor a new model manager.
Abstract base classes
from django.db import models
class TimestampedModel(models.Model):
created_at = models.DateTimeField(auto_now_add=True)
updated_at = models.DateTimeField(auto_now=True)
class Meta:
abstract = True
class Address(TimestampedModel):
state = models.CharField(max_length=10)
country = models.CharField(max_length=10)
pin_code = models.IntegerField()
def __str__(self):
return self.country
When we run makemigrations for the above model changes, only Address model changes will be there. Please note that in the Meta class of TimestampedModel we have put abstract=True, this tells Django, not to create a database table for the corresponding table.
Migration
class Migration(migrations.Migration):
initial = True
dependencies = [
]
operations = [
migrations.CreateModel(
name='Address',
fields=[
('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
('created_at', models.DateTimeField(auto_now_add=True)),
('updated_at', models.DateTimeField(auto_now=True)),
('state', models.CharField(max_length=10)),
('country', models.CharField(max_length=10)),
('pin_code', models.IntegerField()),
],
options={
'abstract': False,
},
),
]
Going forward if we inherit from the TimestampedModel, that would by default add created_at and updated_at, with this, we achieve DRY (Don't Repeat Yourself) and a cleaner way to code.
childClass now has a nice method to soft-delete objects. Let’s see, how it works.
from django.contrib.auth.models import User
from django.db import models
from django.utils import timezone
class BaseSoftDeletableModel(models.Model):
is_deleted = models.BooleanField(default=False)
deleted_at = models.DateTimeField(null=True)
deleted_by = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
class Meta:
abstract = True
def soft_delete(self, user_id=None):
self.is_deleted = True
self.deleted_by = user_id
self.deleted_at = timezone.now()
self.save()
class Address(TimestampedModel, BaseSoftDeletableModel):
state = models.CharField(max=10)
country = models.CharField(max=10)
pin_code = models.IntegerField()
def __str__(self):
return self,country
Now Let’s open the django shell by running command python manage.py shell.
In [1]: from common.models import Address
In [2]: address = Address.objects.create(state='DL', country='IN', pin_code=110011)
In [3]: address.__dict__
Out[3]:
{
'_state': <django.db.models.base.ModelState object at 0x102d573a0>,
'id': 1,
'created_at': datetime.datetime(2022, 7, 29, 13, 11, 30, 557322, tzinfo=<UTC>),
'updated_at': datetime.datetime(2022, 7, 29, 13, 11, 30, 557372, tzinfo=<UTC>),
'is_deleted': False,
'deleted_at': None,
'deleted_by_id': None,
'state': 'DL',
'country': 'IN',
'pin_code': 110011
}
In [4]: address.soft_delete()
In [5]: address.__dict__
Out[5]:
{
'_state': <django.db.models.base.ModelState object at 0x102d573a0>,
'id': 1,
'created_at': datetime.datetime(2022, 7, 29, 13, 11, 30, 557322, tzinfo=<UTC>),
'updated_at': datetime.datetime(2022, 7, 29, 13, 13, 38, 232611, tzinfo=<UTC>),
'is_deleted': True,
'deleted_at': datetime.datetime(2022, 7, 29, 13, 13, 38, 231991, tzinfo=<UTC>),
'deleted_by_id': None,
'state': 'DL',
'country': 'IN',
'pin_code': 110011
}
Multi-table Inheritance
In the above example of Address class, although it is inheriting from multiple parent tables, it is still an example of the abstract base class inheritance style, since both the parent classes are abstract in nature.
For multi-table inheritance, parentClass's table also gets created in the database. The name multi-table comes from the fact that multiple tables actually gets created in the database and not because the childClass is inheriting multiple parentClass.
from django.contrib.auth.models import User
from django.db import models
from django.utils import timezone
class BaseSoftDeletableModel(models.Model):
is_deleted = models.Boolean(default=False)
deleted_at = models.DateTimeField(null=True)
deleted_by = models.ForeignKey(User, null=True)
class Meta:
abstract = True
def soft_delete(self, user_id=None):
self.is_deleted = True
self.deleted_by = user_id
self.deleted_at = timezone.now()
self.save()
class Address(TimestampedModel, BaseSoftDeletableModel):
state = models.CharField(max=10)
country = models.CharField(max=10)
pin_code = models.IntegerField()
def __str__(self):
return self.country
class Place(Address):
name = models.CharField(max_length=10)
def __str__(self):
return self.name
Again, Open the Django Shell python manage.py shell
Now, when we will create a Place instance, it will also create an entry for address in the database.
In [1]: from common.models import Address, Place
In [2]: place = Place.objects.create(name='home', state='KR', country='IN', pin_code=13123)
In [3]: place.__dict__
Out[3]:
{
'_state': <django.db.models.base.ModelState object at 0x103cce9d0>,
'id': 2,
'created_at': datetime.datetime(2022, 7, 29, 13, 18, 6, 259917, tzinfo=<UTC>),
'updated_at': datetime.datetime(2022, 7, 29, 13, 18, 6, 259963, tzinfo=<UTC>),
'is_deleted': False,
'deleted_at': None,
'deleted_by_id': None,
'state': 'DL',
'country': 'IN',
'pin_code': 110022,
'address_ptr_id': 2,
'name': 'home'
}
In [4]: Address.objects.count()
Out[4]: 2
Here you can see, 1 entry for address also got created.
Source Code. github