20
Calling the queryset’s .count() method ultimately calls Count().
Specifically:
django.db.models.QuerySet.count() calls
django.db.models.sql.Query.get_count(), which calls
django.db.models.sql.Query.add_count_column(), which adds
django.db.models.sql.aggregates.Count to the query.
The main difference between the two is that when you use Count directly, you specify the fields you want to count, whereas when you call .count() on the queryset, this will result in SELECT COUNT(*)... (except when you also use distinct() or when you limit the fields in the select clause, in which case it’s more complicated).
13
Apples and oranges. .count() does a SQL count on the current queryset. The Count aggregate, however, runs a count on relationships you specify on the queryset.
Pizza.objects.count() # Total amount of pizzas
Pizza.objects.aggregate(topping_count=Count('toppings')) # Total amount of toppings associated to a pizza
Pizza.objects.annotate(topping_count=Count('toppings')) # Total amount of toppings on each pizza
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