Add workaround for the issue SERVER-114196

Author: WaVEV

django_mongodb_backend/fields/array.py

@@ -320,6 +320,11 @@ def get_subquery_wrapping_pipeline(self, compiler, connection, field_name, expr)
                     "tmp_name": {"$addToSet": "$tmp_name"},
                 }
             },
+            # 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": [{"tmp_name": []}]}]}},
             {"$limit": 1},
             {"$project": {field_name: "$tmp_name"}},

django_mongodb_backend/fields/embedded_model_array.py

@@ -167,6 +167,10 @@ def get_subquery_wrapping_pipeline(self, compiler, connection, field_name, expr)
                     "tmp_name": {"$addToSet": "$tmp_name"},
                 }
             },
+            # 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 a dummy document in case of empty result.
             {"$unionWith": {"pipeline": [{"$documents": [{"tmp_name": []}]}]}},
             {"$limit": 1},

django_mongodb_backend/lookups.py

@@ -62,6 +62,11 @@ def get_subquery_wrapping_pipeline(self, compiler, connection, field_name, expr)
                 "tmp_name": {"$addToSet": expr.as_mql(compiler, connection, as_expr=True)},
             }
         },
+        # 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": [{"tmp_name": []}]}]}},
         {"$limit": 1},
         {"$project": {field_name: "$tmp_name"}},

django_mongodb_backend/query.py

@@ -93,8 +93,16 @@ def get_pipeline(self):
         if self.aggregation_pipeline:
             pipeline.extend(self.aggregation_pipeline)
         if self.wrap_for_global_aggregation:
-            # Add an empty extra document to handle default values on empty results
-            pipeline.append({"$unionWith": {"pipeline": [{"$documents": [{}]}]}})
+            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": [{}]}]}},
+                ]
+            )
         if self.project_fields:
             pipeline.append({"$project": self.project_fields})
         if self.combinator_pipeline:

tests/lookup_/tests.py

@@ -138,6 +138,7 @@ def test_subquery_filter_constant(self):
                         "pipeline": [
                             {"$match": {"num": {"$gt": 2}}},
                             {"$group": {"_id": None, "tmp_name": {"$addToSet": "$num"}}},
+                            {"$addFields": {"__now": "$$NOW"}},
                             {"$unionWith": {"pipeline": [{"$documents": [{"tmp_name": []}]}]}},
                             {"$limit": 1},
                             {"$project": {"num": "$tmp_name"}},