Replace collstats to handle empty results in global aggregations

Author: WaVEV

django_mongodb_backend/query.py

@@ -93,25 +93,8 @@ def get_pipeline(self):
         if self.aggregation_pipeline:
             pipeline.extend(self.aggregation_pipeline)
         if self.wrap_for_global_aggregation:
-            # Use $collStats as a pivot to guarantee a single input document
-            pipeline = [
-                {"$collStats": {}},
-                # Wrap the actual aggregation inside a lookup so its result
-                # always appears as a one document array
-                {
-                    "$lookup": {
-                        "from": self.compiler.collection_name,
-                        "as": "__wrapped",
-                        "pipeline": pipeline,
-                    }
-                },
-                # Use {} If the inner aggregation returns nothing, otherwise unwrap
-                {
-                    "$replaceWith": {
-                        "$cond": [{"$eq": ["$__wrapped", []]}, {}, {"$first": "$__wrapped"}]
-                    }
-                },
-            ]
+            # Add an empty extra document to handle default values on empty results
+            pipeline.append({"$unionWith": {"pipeline": [{"$documents": [{}]}]}})
         if self.project_fields:
             pipeline.append({"$project": self.project_fields})
         if self.combinator_pipeline: