We have an implementation of both annotations in *list_display* and adding 
an aggregates for the entire list (which we call *list_summary* and what 
you are calling here *list_aggregates*) and there are a bunch of subtleties 
in the interaction due to Admin's built in support for pagination and 
filtering

To demonstrate, let's use a simplified use case
assume models such as Author -< Book >- Genre (Book has FKs to both Author 
and Genre)

and and admin such as (as suggested in Stack Overflow post that Tim 
referenced (‚Äčalready possible to some extent  
<http://stackoverflow.com/questions/22517999/django-admin-interface-to-display-aggregates>
)

class AuthorAdmin(admin.ModelAdmin):
    list_display = ('author_name', 'book_count', 'book_sum_price')
    list_filters = ('book__genre__name', )
    list_summary = (('Total Books', Sum('book_count'), ),
                    ('Total Book Price', Sum('book_sum_price'), ))
    
    def get_queryset(self, request):
        return super(BookAdmin, self).get_queryset(request).annotate(
            books_count=Count('books__name'),
            books_sum_price=Sum('books__price'))

With regards to *list_summary* (or *list_aggregates* in the PR) our 
implementation summarizes the entire QuerySet not just a single page (my 
quick read of the patch seems to indicate that list_aggregates only 
aggregates a single page of the qs).  From my perspective summarizing a 
single page doesn't provide as much value summarizing the entire QS.  If 
one agrees  with that then feature will would have to support a user 
friendly name for the field (implemented here as a tuple - as suggested by 
Jim).  Additionally, if the feature summarizes the entire qs then the 
output should likely go above the table rather than as summary below (if it 
were below and the results were paginated, it would likely confuse the 
users)

With regards to extending list_display to support annotations there are 
subtle interactions between admin, annotated query sets and filters.

The qs that Django executes when the user has a filter looks like

Author.objects.annotate(...).filter(...)

In the code shown above this will produce the correct result set, but 
because annotate and filter aren't commutative 
<https://docs.djangoproject.com/en/1.10/topics/db/aggregation/#order-of-annotate-and-filter-clauses>,
 
the generated SQL ends up joining the Books table twice.  Additionally, 
there are probably some cases where complex filters will give unexpected 
results.  

Give that the user really wants

Author.objects.filter(...).annotate(...)

we ended up adding a modelAdmin.get_annotations(...) method and subclassing 
ChangeList to implement this feature.  

One additional note is that annotations require implementing a method on 
the modelAdmin class for each annotations which seems very boilerplate-ish. 
We have extended *list_display* to automatically handle the boilerplate as 
well.  

Since we use this extensively these features in our applications, we would 
be excited to see them implemented as part of admin.  We'd be happy to 
contribute here and if this seems like its worth pursuing I'll ask our team 
to look into refactoring the work we've done so that it could live in core 
rather than as sub-classes.

On Wednesday, September 21, 2016 at 6:17:27 PM UTC-7, Josh Smeaton wrote:
>
> I think I'm OK with `list_aggregates` because it implies a terminal 
> queryset method which really restricts the members used to create that 
> aggregation (the GROUP BY). Adding aggregates to existing list_display 
> would require something *else* to refine the group by using `values()`.
>
> If list_aggregates is a useful feature, then this sounds like an 
> appropriate way to implement that. Regular annotations could be added and 
> processed within list_display, provided list_display was modified to accept 
> expressions (either aggregates or regular annotations) in tuple form for 
> alias support.
>
> list_aggregates -> queryset.aggregate()
> list_display -> queryset.annotate(annotations).values()
>
> Does that make sense?
>

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