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 particul

Answer 1

So long as you order both queries on B and C, it is fairly easy to merge them without having to do an expensive resort:

# first define a couple of helper functions 

def next_or(iterable, other):
    try:
        return iterable.next(), None
    except StopIteration:
        return None, other

def merge(x,y,func=lambda a,b: a<=b):
    ''' merges a pair of sorted iterables '''
    xs = iter(x)
    ys = iter(y)
    a,r = next_or(xs,ys)
    b,r = next_or(ys,xs)
    while r is None:
        if func(a,b):
            yield a
            a,r = next_or(xs,ys)
        else:
            yield b
            b,r = next_or(ys,xs)
    else:
        if a is not None:
            yield a
        else:
            yield b
    for o in r:
        yield o

# now get your objects & then merge them

b_qs = B.objects.filter(NAME__startswith='Z').order_by('ORDER')
c_qs = C.objects.filter(NAME__startswith='Z').order_by('ORDER')

for obj in merge(b_qs,c_qs,lambda a,b: a.ORDER <= b.ORDER):
    print repr(obj), obj.__dict__

The advantage of this technique is it works with an abstract base class.

Answer 2

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 3

Querying using your "b" method, will allow for you to "bring in" all the remaining data without querying your B and C models separately. You can use the "dot lowercase model name" relation.

http://docs.djangoproject.com/en/dev/topics/db/models/#multi-table-inheritance

for object in A.objects.filter(NAME__istartswith='z').order_by('ORDER'):
    if object.b:
        // do something
        pass
    elif object.c:
        // do something
        pass

You may need to try and except DoesNotExist exceptions. I'm a bit rusty with my django. Good Luck.

Answer 4

This question was answered here.

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