diff --git a/docs/topics/db/aggregation.txt b/docs/topics/db/aggregation.txt index 22c4e483df..030335c908 100644 --- a/docs/topics/db/aggregation.txt +++ b/docs/topics/db/aggregation.txt @@ -184,6 +184,8 @@ of the ``annotate()`` clause is a ``QuerySet``; this ``QuerySet`` can be modified using any other ``QuerySet`` operation, including ``filter()``, ``order_by()``, or even additional calls to ``annotate()``. +.. _combining-multiple-aggregations: + Combining multiple aggregations ------------------------------- @@ -340,29 +342,67 @@ Order of ``annotate()`` and ``filter()`` clauses ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When developing a complex query that involves both ``annotate()`` and -``filter()`` clauses, particular attention should be paid to the order -in which the clauses are applied to the ``QuerySet``. +``filter()`` clauses, pay particular attention to the order in which the +clauses are applied to the ``QuerySet``. -When an ``annotate()`` clause is applied to a query, the annotation is -computed over the state of the query up to the point where the annotation -is requested. The practical implication of this is that ``filter()`` and -``annotate()`` are not commutative operations -- that is, there is a -difference between the query:: +When an ``annotate()`` clause is applied to a query, the annotation is computed +over the state of the query up to the point where the annotation is requested. +The practical implication of this is that ``filter()`` and ``annotate()`` are +not commutative operations. - >>> Publisher.objects.annotate(num_books=Count('book')).filter(book__rating__gt=3.0) +Given: -and the query:: +* Publisher A has two books with ratings 4 and 5. +* Publisher B has two books with ratings 1 and 4. +* Publisher C has one book with rating 1. - >>> Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book')) +Here's an example with the ``Count`` aggregate:: -Both queries will return a list of publishers that have at least one good -book (i.e., a book with a rating exceeding 3.0). However, the annotation in -the first query will provide the total number of all books published by the -publisher; the second query will only include good books in the annotated -count. In the first query, the annotation precedes the filter, so the -filter has no effect on the annotation. In the second query, the filter -precedes the annotation, and as a result, the filter constrains the objects -considered when calculating the annotation. + >>> a, b = Publisher.objects.annotate(num_books=Count('book', distinct=True)).filter(book__rating__gt=3.0) + >>> a, a.num_books + (, 2) + >>> b, b.num_books + (, 2) + + >>> a, b = Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book')) + >>> a, a.num_books + (, 2) + >>> b, b.num_books + (, 1) + +Both queries return a list of publishers that have at least one book with a +rating exceeding 3.0, hence publisher C is excluded. + +In the first query, the annotation precedes the filter, so the filter has no +effect on the annotation. ``distinct=True`` is required to avoid a +:ref:`cross-join bug `. + +The second query counts the number of books that have a rating exceeding 3.0 +for each publisher. The filter precedes the annotation, so the filter +constrains the objects considered when calculating the annotation. + +Here's another example with the ``Avg`` aggregate:: + + >>> a, b = Publisher.objects.annotate(avg_rating=Avg('book__rating')).filter(book__rating__gt=3.0) + >>> a, a.avg_rating + (, 4.5) # (5+4)/2 + >>> b, b.avg_rating + (, 2.5) # (1+4)/2 + + >>> a, b = Publisher.objects.filter(book__rating__gt=3.0).annotate(avg_rating=Avg('book__rating')) + >>> a, a.avg_rating + (, 4.5) # (5+4)/2 + >>> b, b.avg_rating + (, 4.0) # 4/1 (book with rating 1 excluded) + +The first query asks for the average rating of all a publisher's books for +publisher's that have at least one book with a rating exceeding 3.0. The second +query asks for the average of a publisher's book's ratings for only those +ratings exceeding 3.0. + +It's difficult to intuit how the ORM will translate complex querysets into SQL +queries so when in doubt, inspect the SQL with ``str(queryset.query)`` and +write plenty of tests. ``order_by()`` --------------