How can I count related objects in Django (in less than N queries, where N is number of object).
To clarify, let's say I have tables A and B. Every B is connected to exactly one A. Approach I tried:
A.objects.select_related().filter(attr=val)
A[i].B_set.count()
Of course, for every A[i] I want to find out number of B objects Django executes one query.
So the question is - is there a way to optimize that?
I have not tried how many queries are executed, but the Django way should be using annotate(). For example:
from django.db.models import Count
q = A.objects.select_related('B').annotate(num_B=Count('B'))
print A[0].num_B
Since Django 2.0 Count() aggregate function accepts filter parameter, which allows applying additional restrictions on the related objects queryset.
Works like this:
A.objects.select_related().annotate(
total=models.Count('myrelated__pk', filter=Q(only_items='that-i-need'))
).all()
I have to answer my own question :) If object of A is queried something like this:
A.objects.select_related().filter(atrr=val).annotate(n_b=models.Count('B'))
This creates very long query, but at least there is just one.
