Remove $facet in top level group stages

Author: WaVEV

django_mongodb_backend/aggregates.py

@@ -52,6 +52,7 @@ def count(self, compiler, connection, resolve_inner_expression=False):
     # If distinct=True or resolve_inner_expression=False, sum the size of the
     # set.
     lhs_mql = process_lhs(self, compiler, connection, as_expr=True)
+    lhs_mql = {"$ifNull": [lhs_mql, []]}
     # None shouldn't be counted, so subtract 1 if it's present.
     exits_null = {"$cond": {"if": {"$in": [{"$literal": None}, lhs_mql]}, "then": -1, "else": 0}}
     return {"$add": [{"$size": lhs_mql}, exits_null]}

django_mongodb_backend/compiler.py

@@ -235,20 +235,10 @@ def _build_aggregation_pipeline(self, ids, group):
         pipeline = []
         if not ids:
             group["_id"] = None
-            pipeline.append({"$facet": {"group": [{"$group": group}]}})
-            pipeline.append(
-                {
-                    "$addFields": {
-                        key: {
-                            "$getField": {
-                                "input": {"$arrayElemAt": ["$group", 0]},
-                                "field": key,
-                            }
-                        }
-                        for key in group
-                    }
-                }
-            )
+            pipeline.append({"$group": group})
+            # It may be a bug, $$NOW has to be called to be reachable in the rest of the pipeline.
+            pipeline.append({"$set": {"__now": "$$NOW"}})
+            pipeline.append({"$unionWith": {"pipeline": [{"$documents": [{}]}]}})
         else:
             group["_id"] = ids
             pipeline.append({"$group": group})