Django: How can I implement Hierarchy of classes in Django?

Question

I am completely aware of MVC Framework and how Django implements models and views. What I want to know how can I implement custom Hierarchy classes and then use them in Django. For instance:

There is an abstract Class Employee and then subclasses; Permanent Employee,Interns etc. An employee can be hired and fired by the company.

Answer 1

Model inheritance in Django works almost identically to the way normal class inheritance works in Python, but the basics at the beginning of the page should still be followed. That means the base class should subclass django.db.models.Model.

The only decision you have to make is whether you want the parent models to be models in their own right (with their own database tables), or if the parents are just holders of common information that will only be visible through the child models.

There are three styles of inheritance that are possible in Django.

1)

Often, you will just want to use the parent class to hold information that you don’t want to have to type out for each child model. This class isn’t going to ever be used in isolation, so Abstract base classes are what you’re after.

from django.db import models

class CommonInfo(models.Model):
    name = models.CharField(max_length=100)
    age = models.PositiveIntegerField()

    class Meta:
        abstract = True

class Student(CommonInfo):
    home_group = models.CharField(max_length=5)

Edit:

Abstract base classes

Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put abstract=True in the Meta class. This model will then not be used to create any database table. Instead, when it is used as a base class for other models, its fields will be added to those of the child class. It is an error to have fields in the abstract base class with the same name as those in the child (and Django will raise an exception).

from django.db import models

class CommonInfo(models.Model):
    name = models.CharField(max_length=100)
    age = models.PositiveIntegerField()

    class Meta:
        abstract = True

class Student(CommonInfo):
    home_group = models.CharField(max_length=5)

The Student model will have three fields: name, age and home_group. The CommonInfo model cannot be used as a normal Django model, since it is an abstract base class. It does not generate a database table or have a manager, and cannot be instantiated or saved directly.

For many uses, this type of model inheritance will be exactly what you want. It provides a way to factor out common information at the Python level, while still only creating one database table per child model at the database level.

2)

If you’re subclassing an existing model (perhaps something from another application entirely) and want each model to have its own database table, Multi-table inheritance is the way to go.

from django.db import models

class CommonInfo(models.Model):
    # ...
    class Meta:
        abstract = True
        ordering = ['name']

class Student(CommonInfo):
    # ...
    class Meta(CommonInfo.Meta):
        db_table = 'student_info'

3)

Finally, if you only want to modify the Python-level behavior of a model, without changing the models fields in any way, you can use Proxy models.

from django.db import models

class Base(models.Model):
    m2m = models.ManyToManyField(
        OtherModel,
        related_name="%(app_label)s_%(class)s_related",
        related_query_name="%(app_label)s_%(class)ss",
    )

    class Meta:
        abstract = True

class ChildA(Base):
    pass

class ChildB(Base):
    pass

Along with another app rare/models.py:

from common.models import Base

class ChildB(Base):
    pass

For more information you may want to continue reading the documentation here.

---

There is a well maintenanced project, called django-mptt, that implementing Modified Preorder Tree Traversal with your Django Models and working with trees of Model instances.