Count Aggregation documentation

[ebertti@…]

add something in the documentation talking about the possibility of using distinct aggregation done by count.

like:

Number of authors by category:

Author.objects.values('post__category__name').aggregate(qtd=Count('id', distinct=True))

There is nothing on the internet or in the documentation and talking something about it. I find that looking at the code.

[Aymeric Augustin]

Internal APIs aren't documented because they're subject to change without warning.

Could you check if the following query — which only uses documented APIs — achieves the same result?

Author.objects.values('post__category__name').distinct('id').aggregate(qtd=Count('id'))

[anonymous]

Hi!

distinct by column name isn't suported by mysql backend.

[Russell Keith-Magee]

I'm comfortable calling this an oversight on my part. SELECT COUNT(DISTINCT foo) FROM table is different to SELECT DISTINCT COUNT(foo) FROM table, and support for distinct counts was a deliberate decision.

Although it's a feature that may not be universally available on all databases, it is useful on those databases that support it.

[jorgecarleitao]

This can be documented when ticket #14030 is addressed.

[jorgecarleitao]

Forget previous comment.

Isn't this documented in Count, ​https://docs.djangoproject.com/en/1.6/ref/models/querysets/#django.db.models.Count.distinct, which is different from distinct, ​https://docs.djangoproject.com/en/1.6/ref/models/querysets/#distinct ?

[Tim Graham]

Yep, that looks like it.