Maximum of an annotation after a group by

Question

I would like to compute the maximum of \"a_priority\" for each group of (b, c) pairs.

a_priority is an annotation based on a case/when mapping strings to priority va

Answer 1

Have you tried putting Case expression directly into Max? It is possible since Django 1.8.

from django.db.models import Max, Case, When, IntegerField
qs = MyObject.objects.all()
a_priority=Case(
    When(a='A', then=1), 
    When(a='S', then=2),
    When(a='Q', then=3),        
    output_field=IntegerField()
)
qs = qs.values("b", "c").annotate(max_a_priority=Max(a_priority))

Answer 2

I believe the qs.values("b", "c") filters out my annotation a_priority.

You are correct! values constrain the columns that are returned in the resulting QuerySet. But since the QuerySet is linked to the original model, you can filter using columns not provided in values (ex: qs.values('b', 'c').filter('a'='A')).

The best way to accomplish your task is Piotr Ćwiek's answer. But to make the answers more comprehensive (and to get to know aggregations better):

• explicitly include a_priority in your values:

  qs1 = qs.values("b", "c", "a_priority").annotate(max_priority=Max("a_priority")).distinct()
  # Or
  qs2 = qs.values("b", "c", "a_priority").annotate(max_priority=Max("a_priority")).filter(a_priority=F('max_a_priority'))
  

• reverse the order of values and annotate:

  qs3 = qs.annotate(max_priority=Max("a_priority")).values("b", "c").distinct()
  

Django docs explain why distinct (or filter) is required here, and not in Piotr's answer:

If the values() clause precedes the annotate(), the annotation will be computed using the grouping described by the values() clause.

So a_priority is also included in the GROUP BY clause in qs1 and qs2, hence distinct is required.

However, if the annotate() clause precedes the values() clause, the annotations will be generated over the entire query set. In this case, the values() clause only constrains the fields that are generated on output.

This explains qs3. Internally, Django does GROUP BY by id to get results over the entire QuerySet.