Vicente G. Reyes

Posted on Mar 1, 2023 • Updated on May 8, 2023 • Originally published at vicentereyes.org

Django Model Inheritance

Django is a popular web framework that allows developers to build web applications quickly and easily. One of the powerful features of Django is its support for model inheritance. Model inheritance enables developers to create new models by inheriting fields from existing models. This feature can be used to build complex data models that are easy to maintain and extend. In this article, we will explore Django model inheritance and its various types.

Types of Model Inheritance

Abstract Base Classes
Multi-table Inheritance
Proxy Models

Abstract Base Classes

Abstract Base Classes (ABCs) are model classes that cannot be instantiated. They are used as base classes for other models and provide common fields and methods that can be reused by other models. ABCs can be created using the abstract attribute in the model's Meta class.

from django.db import models

class AbstractPerson(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    email = models.EmailField()

    class Meta:
        abstract = True

class Employee(AbstractPerson):
    job_title = models.CharField(max_length=100)
    salary = models.IntegerField()

class Customer(AbstractPerson):
    address = models.CharField(max_length=200)
    phone_number = models.CharField(max_length=20)

In the above example, AbstractPerson is an ABC that defines the common fields for Employee and Customer. Employee and Customer inherit the fields from AbstractPerson and add their own fields.

Multi-table Inheritance

Multi-table inheritance is a type of inheritance in which each model in the inheritance hierarchy is mapped to its own database table. This allows each model to have its own fields and methods, but also share the fields and methods of its parent model. Multi-table inheritance can be created by defining a model that inherits from another model.

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    email = models.EmailField()

class Employee(Person):
    job_title = models.CharField(max_length=100)
    salary = models.IntegerField()

class Customer(Person):
    address = models.CharField(max_length=200)
    phone_number = models.CharField(max_length=20)

In the above example, Employee and Customer inherit from Person. Each model has its own table in the database, but they share the fields defined in Person.

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    email = models.EmailField()

class Employee(Person):
    job_title = models.CharField(max_length=100)
    salary = models.IntegerField()

    class Meta:
        proxy = True

class Manager(Employee):
    class Meta:
        proxy = True

    def get_employees(self):
        return Employee.objects.filter(job_title='Manager', salary__gt=self.salary)

In the above example, Employee is a concrete model that inherits from Person. Manager is a proxy model that inherits from Employee. Manager does not add any new fields, but it does add a new method, get_employees(), that returns all the employees with the job title of "Manager" and a higher salary than the manager.