Fixed #31880 -- Made QuerySet.aggregate() raise FieldError when aggregating over aggregation aliases.

[David-Wobrock]

Ticket: https://code.djangoproject.com/ticket/31880

(First implementation: #13390)

Suggested patch for the discussion https://code.djangoproject.com/ticket/31880#comment:7

[felixxm]

@jacobtylerwalls proposed the same solution to fix ticket-31679, see #13273. My initial response was that it's too restrictive. I was thinking about checking that an aggregation overrides and uses the same aliases 🤔. If it's not feasible or it's too complicated then I'm fine with this patch but we should add a backward incompatible note.

[David-Wobrock]

Hi @felixxm
The patch is indeed exactly the same :P And I agree that it is too restrictive.
I tried to focus back on the issue at hand:
Considering Author.objects.annotate(age_alias=F('age')).aggregate(age=Sum(F('age_alias')), avg_age=Avg(F('age'))), the last aggregation is referring to the aggregation made just before, in the same aggregate call.

This should be less restrictive and still allow currently working QuerySets like Author.objects.annotate(age_alias=F('age')).aggregate(age_alias=Sum(F('age_alias')).
But still prevent the initial problem presented in the ticket https://code.djangoproject.com/ticket/31880

What do you think about this approach? 🤔

[felixxm]

@David-Wobrock Looks good, thanks 👍

[David-Wobrock]

@felixxm Merged the suggestions into one commit, thanks!

[felixxm]

@David-Wobrock Thanks 👍