Fix subquery computed twice.

Author: WaVEV

django_mongodb/compiler.py

@@ -401,19 +401,15 @@ def build_query(self, columns=None):
         else:
             # If we query distinct
             if self.query.distinct:
-                distinct_fields = self.get_project_fields(columns, force_expression=True)
+                distinct_fields = self.get_project_fields(
+                    columns, ordering_fields, force_expression=True
+                )
                 if not query.aggregation_pipeline:
                     query.aggregation_pipeline = []
-                query.aggregation_pipeline.extend(
-                    [
-                        {"$group": {"_id": distinct_fields}},
-                        {"$addFields": {key: f"$_id.{key}" for key in distinct_fields}},
-                    ]
-                )
-                if "_id" not in distinct_fields:
-                    query.aggregation_pipeline.append({"$unset": "_id"})
-
-            query.project_fields = self.get_project_fields(columns, ordering_fields)
+                query.aggregation_pipeline.append({"$group": {"_id": distinct_fields}})
+                query.project_fields = {key: f"$_id.{key}" for key in distinct_fields}
+            else:
+                query.project_fields = self.get_project_fields(columns, ordering_fields)
             # If columns is None, then get_project_fields() won't add
             # ordering_fields to $project. Use $addFields (extra_fields) instead.
             if columns is None: