Fetching inherited model objects in django

Question

I have a django application with the following model:

Object A is a simple object extending from Model with a few fields, and let's say, a particular one is a char field called "NAME" and an Integer field called "ORDER". A is abstract, meaning there are no A objects in the database, but instead...

Objects B and C are specializations of A, meaning they inherit from A and they add some other fields.

Now suppose I need all the objects whose field NAME start with the letter "Z", ordered by the ORDER field, but I want all the B and C-specific fields too for those objects. Now I see 2 approaches:

a) Do the queries individually for B and C objects and fetch two lists, merge them, order manually and work with that.

b) Query A objects for names starting with "Z" ordered by "ORDER" and with the result query the B and C objects to bring all the remaining data.

Both approaches sound highly inefficient, in the first one I have to order them myself, in the second one I have to query the database multiple times.

Is there a magical way I'm missing to fetch all B and C objects, ordered in one single method? Or at least a more efficient way to do this than the both mentioned?

Thanks in Advance!

Bruno

Answer 1

If A can be concrete, you can do this all in one query using select_related.

from django.db import connection
q = A.objects.filter(NAME__istartswith='z').order_by('ORDER').select_related('b', 'c')
for obj in q:
   obj = obj.b or obj.c or obj
   print repr(obj), obj.__dict__ # (to prove the subclass-specific attributes exist)
print "query count:", len(connection.queries)

Answer 2

This question was answered here.

Use the InheritanceManager from the django-model-utils project.