Count() in QuerySet.annotate() crashes: Unknown expression $count

[timgraham]

QuerySet: Author.objects.annotate(c=Count("id"))

SQL:

SELECT "annotations_author"."id",
       "annotations_author"."name",
       "annotations_author"."age",
       COUNT("annotations_author"."id") AS "c"
FROM "annotations_author"
GROUP BY "annotations_author"."id",
         "annotations_author"."name",
         "annotations_author"."age"

MQL:
{'$project': {'_id': 1, 'name': 1, 'age': 1, 'c': {'$count': ['$_id']}}}
Error:
pymongo.errors.OperationFailure: Invalid $project :: caused by :: Unknown expression $count,

If the query can't be expressed correctly, django-mongodb should raise a helpful NotSupportedError, and the limitation should be documented in the README.

[WaVEV]

Maybe fixing #12 will fix this one too.

[timgraham]

Fixed by #84: add support for QuerySet.aggregate().