Aggregating annotations raises DatabaseError

[Debanshu Kundu]

If I have a model Book defined as:

class Book(models.Model):
   name = models.CharField(max_length=300)
   pages = models.IntegerField()
   price = models.DecimalField(max_digits=10, decimal_places=2)
   rating = models.FloatField()
   pubdate = models.DateField()

and I run the query:

Book.objects.values('rating').annotate(books_per_rating=Count('id')).aggregate(Max('books_per_rating'))

I get a DatabaseError.

According to this ​https://docs.djangoproject.com/en/1.5/topics/db/aggregation/#aggregating-annotations, Django supports aggregating annotations. But in the example given in the link itself, they annotate over a QuerySet which in turn returns a QuerySet (and not ValuesQuerySet), so aggregate method runs successfully. But in my example aggregating a ValuesQuerySet raises a DatabaseError.

Is it a bug in Django? Because if Django does not support aggregation over ValuesQuerySet then it should raise an exception at Django level (not DatabaseError).

Also interestingly, if I do:

Book.objects.values('rating').aggregate(Max('rating'))

It returns an empty dict without performing any DB query!!

Note: Error was raised on 'postgresql_psycopg2' and 'sqlite3' backends. Haven't been tested on other backends.

[Anssi Kääriäinen <akaariai@…>]

In 4bd55547214ae149acadffe60536c379d51e318f:

Fixed #20782 -- qs.values().aggregate() failure

In the combination of .values().aggregate() the aggregate_select_mask
didn't include the aggregates added. This resulted in bogus query.

Thanks to Trac alias debanshuk for report.