Can't perform annotation on related table when relation between tables not on primary key

[powderflask]

I'm working with a legacy DB (ArcSDE database -- really ugly) -- have been able to accommodate its oddities without resorting much to raw SQL, but django upgrade (1.8 --> 1.11) caused a previously working annotation to fail:
psycopg2.ProgrammingError: :column ... must appear in the GROUP BY clause or be used in an aggregate function

DB: PostgreSQL 9.3.16 (i.e., this is not same issue as #26758 ) Python3.6, Django1.11

The annotation simply counts the number of related records from a related 'attachments' table:
qs.annotate(num_attachments=Count('attachments'))

The root cause appears to be that the relation between the model and attachments tables uses a unique field other than the main model's primary key (I know -- told you it was ugly - ArcSDE does not really support relations, except they implement attachments with this odd ball sigh ).
The change in behaviour seems to manifest from #19259 (I believe django1.8 added all fields to the groupby clause).
Since django now includes only the primary key in the groupby clause, postgresql won't do this aggregation across a relation that uses a non-pk field.

I suspect there is a general issue here that aggregations on a related-table won't work in postgresql unless the relation is on the primary key field (so, yeah, basically this issue applies to almost no one, right...).

Seems likely there is a better solution to this, but after a half-day of search / effort, I resorted to the following:

qs.extra(select={'num_attachments':
            'SELECT COUNT("attachmentid") FROM {attach_table} WHERE {attach_table}.rel_globalid = {model_table}.globalid'.format(
                    model_table  = qs.model._meta.db_table,
                    attach_table = qs.model.AttachmentModel._meta.db_table,
            )},)

This works and achieves my goal -- to annotate model with the number of related attachments.
Since the model.attachments.count() query works just fine, I'm considering eliminating this annotation and replacing with a property on the model class, what's one more query... I'm sure there must be an smoother way, but it eludes me...

Since the docs suggested to open a ticket for queries that could not be resolved without resorting to extra(), here it is, for whatever its worth. Hope this hasn't been a complete waste of time for you.