Filtering the Aggregate in the Django ORM

Question

I have a function that looks like this:

def post_count(self):
        return self.thread_set.aggregate(num_posts=Count(\'post\'))[\'num_posts\']

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Answer 1

OK, now that the question includes the model definitions, I submit to you that this should work, unless your version of Django doesn't support some feature I use here (in which case, please let me know!):

Post.objects.filter(thread__in=thread_set, status='active').aggregate(num_posts=Count('id'))

Django allows __in filters to take a QuerySet to decide what the IN clause should look like in SQL, so if you pass thread__in=thread_set, Django will filter the posts so that only those whose thread field points to one of the ids of the threads in your thread_set remain for the aggregate call to see.

This should filter the posts with just one db query with something like WHERE thread_id IN ... inside, rather than with one query per thread, which would indeed be horrid. If anything else happened, this would be a bug in Django...

The result should be at most two queries to establish a Category's postcount -- one to obtain thread_set and another one actually to count the posts. The alternative is to have a thread/post join to be filtered based on Thread's category field and Post's status field, which I wouldn't necessarily expect to be that much faster. (I say 'at most', because I guess they could be fused automatically... Though I don't think this would happen with current Django. Can't check ATM, sorry.)

EDIT: Django's QuerySet API reference says this on __in filters:

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IN

In a given list.

Example:

Entry.objects.filter(id__in=[1, 3, 4])

SQL equivalent:

SELECT ... WHERE id IN (1, 3, 4);

You can also use a queryset to dynamically evaluate the list of values instead of providing a list of literal values:

inner_qs = Blog.objects.filter(name__contains='Cheddar')
entries = Entry.objects.filter(blog__in=inner_qs)

This queryset will be evaluated as subselect statement:

SELECT ... WHERE blog.id IN (SELECT id FROM ... WHERE NAME LIKE '%Cheddar%')

The above code fragment could also be written as follows:

inner_q = Blog.objects.filter(name__contains='Cheddar').values('pk').query
entries = Entry.objects.filter(blog__in=inner_q)

Changed in Django 1.1: In Django 1.0, only the latter piece of code is valid.

This second form is a bit less readable and unnatural to write, since it accesses the internal query attribute and requires a ValuesQuerySet. If your code doesn't require compatibility with Django 1.0, use the first form, passing in a queryset directly.

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So, I guess Django is capable of passing a single query to the db in the case at issue here. If the db's query analyser does a good job, the effect might be very nearly optimal. :-)