EDIT:
the query should be:
Book.objects.values('rating').annotate(books_per_rating=Count('id')).aggregate(Max('books_per_rating'))
*books_per_rating* should be quoted in *Max* call.
On Friday, July 19, 2013 11:49:52 AM UTC+5:30, Debanshu Kundu wrote:
>
> 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*).
>
>
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