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

@@ -38,6 +38,7 @@ def __init__(self, *args, **kwargs):
         self.subqueries = []
         # Atlas search stage.
         self.search_pipeline = []
+        self.wrap_for_global_aggregation = False
 
     def _get_group_alias_column(self, expr, annotation_group_idx):
         """Generate a dummy field for use in the ids fields in $group."""
@@ -234,21 +235,8 @@ def _build_aggregation_pipeline(self, ids, group):
         """Build the aggregation pipeline for grouping."""
         pipeline = []
         if not ids:
-            group["_id"] = None
-            pipeline.append({"$facet": {"group": [{"$group": group}]}})
-            pipeline.append(
-                {
-                    "$addFields": {
-                        key: {
-                            "$getField": {
-                                "input": {"$arrayElemAt": ["$group", 0]},
-                                "field": key,
-                            }
-                        }
-                        for key in group
-                    }
-                }
-            )
+            pipeline.append({"$group": {"_id": None, **group}})
+            self.wrap_for_global_aggregation = True
         else:
             group["_id"] = ids
             pipeline.append({"$group": group})

django_mongodb_backend/query.py

@@ -56,6 +56,7 @@ def __init__(self, compiler):
         # $lookup stage that encapsulates the pipeline for performing a nested
         # subquery.
         self.subquery_lookup = None
+        self.wrap_for_global_aggregation = compiler.wrap_for_global_aggregation
 
     def __repr__(self):
         return f"<MongoQuery: {self.match_mql!r} ORDER {self.ordering!r}>"
@@ -91,6 +92,22 @@ def get_pipeline(self):
             pipeline.append({"$match": self.match_mql})
         if self.aggregation_pipeline:
             pipeline.extend(self.aggregation_pipeline)
+        if self.wrap_for_global_aggregation:
+            pipeline = [
+                {"$collStats": {}},
+                {
+                    "$lookup": {
+                        "from": self.compiler.collection_name,
+                        "as": "wrapped",
+                        "pipeline": pipeline,
+                    }
+                },
+                {
+                    "$replaceWith": {
+                        "$cond": [{"$eq": ["$wrapped", []]}, {}, {"$first": "$wrapped"}]
+                    }
+                },
+            ]
         if self.project_fields:
             pipeline.append({"$project": self.project_fields})
         if self.combinator_pipeline: