Django ORM: Selecting related set

Question

Say I have 2 models:

class Poll(models.Model):     category = models.CharField(u"Category", max_length = 64)     [...]  class Choice(models.Model):     poll = models.ForeignKey(Poll)     [...] 

Given a Poll object, I can query its choices with:

poll.choice_set.all() 

But, is there a utility function to query all choices from a set of Poll?

Actually, I'm looking for something like the following (which is not supported, and I don't seek how it could be):

polls = Poll.objects.filter(category = 'foo').select_related('choice_set') for poll in polls:     print poll.choice_set.all() # this shouldn't perform a SQL query at each iteration 

I made an (ugly) function to help me achieve that:

def qbind(objects, target_name, model, field_name):     objects = list(objects)     objects_dict = dict([(object.id, object) for object in objects])     for foreign in model.objects.filter(**{field_name + '__in': objects_dict.keys()}):         id = getattr(foreign, field_name + '_id')         if id in objects_dict:             object = objects_dict[id]             if hasattr(object, target_name):                 getattr(object, target_name).append(foreign)             else:                 setattr(object, target_name, [foreign])     return objects 

which is used as follow:

polls = Poll.objects.filter(category = 'foo') polls = qbind(polls, 'choices', Choice, 'poll') # Now, each object in polls have a 'choices' member with the list of choices. # This was achieved with 2 SQL queries only. 

Is there something easier already provided by Django? Or at least, a snippet doing the same thing in a better way.

How do you handle this problem usually?

Answer 1

Time has passed and this functionality is now available in Django 1.4 with the introduction of the prefetch_related() QuerySet function. This function effectively does what is performed by the suggested qbind function. ie. Two queries are performed and the join occurs in Python land, but now this is handled by the ORM.

The original query request would now become:

polls = Poll.objects.filter(category = 'foo').prefetch_related('choice_set') 

As is shown in the following code sample, the polls QuerySet can be used to obtain all Choice objects per Poll without requiring any further database hits:

for poll in polls:     for choice in poll.choice_set:         print choice