INTPYTHON-833 Remove $facet when an ArrayField is used as rhs

Author: WaVEV

django_mongodb_backend/aggregates.py

@@ -7,6 +7,7 @@
     Variance,
 )
 from django.db.models.expressions import Case, Value, When
+from django.db.models.functions.comparison import Coalesce
 from django.db.models.lookups import IsNull
 from django.db.models.sql.where import WhereNode
 
@@ -69,6 +70,8 @@ def count(self, compiler, connection, resolve_inner_expression=False):
         if resolve_inner_expression:
             return lhs_mql
         return {"$sum": lhs_mql}
+    # Normalize empty documents (introduced upstream) to an empty set fallback.
+    agg_expression = Coalesce(agg_expression, [])
     # If distinct=True or resolve_inner_expression=False, sum the size of the
     # set.
     return {"$size": agg_expression.as_mql(compiler, connection, as_expr=True)}

django_mongodb_backend/compiler.py

@@ -38,6 +38,10 @@ def __init__(self, *args, **kwargs):
         self.subqueries = []
         # Atlas search stage.
         self.search_pipeline = []
+        # Does the aggregation have no GROUP BY fields and needs wrapping?
+        self.wrap_for_global_aggregation = False
+        # The MQL equivalent to a SQL HAVING clause.
+        self.having_match_mql = None
 
     def _get_group_alias_column(self, expr, annotation_group_idx):
         """Generate a dummy field for use in the ids fields in $group."""
@@ -234,21 +238,10 @@ def _build_aggregation_pipeline(self, ids, group):
         """Build the aggregation pipeline for grouping."""
         pipeline = []
         if not ids:
-            group["_id"] = None
-            pipeline.append({"$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}})
+            # Apply a global aggregation if there are no ids and no having
+            # clause.
+            self.wrap_for_global_aggregation = not bool(self.having)
         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 +92,23 @@ 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.extend(
+                [
+                    # Workaround for https://jira.mongodb.org/browse/SERVER-114196:
+                    # $$NOW becomes unavailable after $unionWith, so it must be
+                    # stored beforehand to ensure it remains accessible later
+                    # in the pipeline.
+                    {"$addFields": {"__now": "$$NOW"}},
+                    # Add an empty extra document to handle default values on
+                    # empty results.
+                    {"$unionWith": {"pipeline": [{"$documents": [{}]}]}},
+                    # Limiting to one document ensures the original result
+                    # takes precedence when present, otherwise the injected
+                    # empty document is used.
+                    {"$limit": 1},
+                ]
+            )
         if self.project_fields:
             pipeline.append({"$project": self.project_fields})
         if self.combinator_pipeline:

docs/releases/6.0.x.rst

@@ -17,6 +17,11 @@ Bug fixes
 
 - ...
 
+Performance improvements
+------------------------
+
+- Removed usage of ``$facet`` from aggregate queries.
+
 6.0.0
 =====