Question

In Django, I have the following models.py

class Product(RandomPrimaryIdModel):
  feature1 = models.CharField(max_length=20, blank=True, null=True)
  feature2 = models.CharField(max_length=20, blank=True, null=True)
  feature3 = models.CharField(max_length=20, blank=True, null=True)

class Mattress(Product):
  category_type = models.CharField(max_length=50)
  size = models.CharField(max_length=5)

  def category(self):
    return "bedding"
  category = property(category)

I have the following views.py file

def update(request, id):
  product = Product.objects.get(id=id)
  ...

In this method, update, can I call a method defined in the “Mattress” model from the Product model. For example, I want to write: if product.type == “mattress” where type has been defined in the Mattress Model and Mattress is a sub-model of Product.

Answer 1

Your example seems to sit between two different ways you can go, but is currently not correct. What is happening is that you are creating two tables: Product, and Mattress, and they are completely unrelated. Regardless of the fact that Mattress subclasses Product, it is just inheriting its structure. You cannot query anything in the Product table about a mattress because a mattress is in the Mattress table.

One way to go is to consider a Product just abstract, to be subclassed by actual products:

class Product(RandomPrimaryIdModel):
    class Meta:
        abstract=True

This will prevent a Product table from being created. Then you would directly query a mattress via: Mattress.objects.filter()

But this seems a bit limiting in terms of introducing many types of products, and having to manage different tables for them. The other way to go is to use a Product table, but use generic relations to support attaching any type of other table as a content object:

from django.db import models
from django.contrib.contenttypes.models import ContentType
from django.contrib.contenttypes import generic

class Product(RandomPrimaryIdModel):

    feature1 = models.CharField(max_length=20, blank=True, null=True)
    feature2 = models.CharField(max_length=20, blank=True, null=True)
    feature3 = models.CharField(max_length=20, blank=True, null=True)

    content_type = models.ForeignKey(ContentType)
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_type', 'object_id')

With this, you would be able to set the content_object to be a Mattress instance. You can then use the ContentType to query:

p_type = ContentType.objects.get(name="mattress")
Product.objects.filter(content_type=p_type)