Using django's ORM `annotate()` and/or `aggregate()`: I want to sum up based on one category field and then average over the category values per date. I tried to do it using two `annotate()` statements but got a `FieldError`.

I'm doing this:

```
queryset1 = self.data.values('date', 'category').annotate(sum_for_field=Sum('category'))

```

Which outputs a `ValuesQuerySet` object with things like (so a sum for each value of category):

```
[{'category': 'apples', 'date': '2015-10-12', sum_for_field=2000},
 {'category': 'carrots', 'date': '2015-10-12', sum_for_field=5000},
 {'category': 'apples', 'date': '2015-10-13', sum_for_field=3000},
 {'category': 'carrots', 'date': '2015-10-13', sum_for_field=6000}, ...
]

```

I then want to average the `sum_for_field` field for each date to output something like:

```
[ {'date': '2015-10-12', avg_final: 3500},
{'date': '2015-10-13', avg_final: 4500}, ...
]

```

I tried doing this:

```
queryset2 = queryset1.values('date', 'sum_for_field')
result = queryset2.annotate(avg_final=Avg('sum_for_field'))

```

But I got this `FieldError`:

```
FieldError: FieldError: Cannot compute Avg('sum_for_field'): 'sum_for_field' is an aggregate

```