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

Welcome to Django model inheritance. Enjoy(!) your stay. – Ignacio Vazquez-Abrams Mar 03 '11 at 02:31 — Ignacio Vazquez-Abrams, Mar 03 '11 at 02:31

DrMeers · Accepted Answer · 2011-03-03T09:27:43.530

6

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)

edited Mar 03 '11 at 09:27

answered Mar 03 '11 at 06:57

DrMeers

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2

Clever. Horrible, but clever. Won't work when A is abstract though? – Wogan Mar 03 '11 at 07:39
Correct, concrete multi-table inheritance only. – DrMeers Mar 03 '11 at 07:41
I agree with Wogan, conceptually it's quite ugly, but it works like a charm providing we can make A concrete, which is not such a terrible thing to do in my case! Very Clever! – Bruno Mar 03 '11 at 17:07

score 1 · Answer 2 · edited May 23 '17 at 11:56

1

This question was answered here.

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

edited May 23 '17 at 11:56

Community

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answered Aug 08 '13 at 14:30

Philipp Zedler

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score 0 · Answer 3 · answered Mar 03 '11 at 05:42

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.

Yes, you would need to catch `DoesNotExist`, and each attempt to access a related object (successful or not) will cost you an additional query -- terribly inefficient. — DrMeers, Mar 03 '11 at 07:03

score 0 · Answer 4 · answered Mar 03 '11 at 08:28

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.

Fetching inherited model objects in django

4 Answers4

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