Fix aggregate pre-pipeline overwrited.

Author: WaVEV

django_mongodb/expressions.py

@@ -119,37 +119,43 @@ def query(self, compiler, connection, lookup_name=None):
     }
     # The result must be a list of values. The output is compressed with an aggregation pipeline.
     if lookup_name in ("in", "range"):
-        subquery.aggregation_pipeline = [
-            {
-                "$facet": {
-                    "group": [
-                        {
-                            "$group": {
-                                "_id": None,
-                                "tmp_name": {
-                                    "$addToSet": expr.as_mql(subquery_compiler, connection)
-                                },
-                            }
-                        }
-                    ]
-                }
-            },
-            {
-                "$project": {
-                    field_name: {
-                        "$ifNull": [
+        if subquery.aggregation_pipeline is None:
+            subquery.aggregation_pipeline = []
+        subquery.aggregation_pipeline.extend(
+            [
+                {
+                    "$facet": {
+                        "group": [
                             {
-                                "$getField": {
-                                    "input": {"$arrayElemAt": ["$group", 0]},
-                                    "field": "tmp_name",
+                                "$group": {
+                                    "_id": None,
+                                    "tmp_name": {
+                                        "$addToSet": expr.as_mql(subquery_compiler, connection)
+                                    },
                                 }
-                            },
-                            [],
+                            }
                         ]
                     }
-                }
-            },
-        ]
+                },
+                {
+                    "$project": {
+                        field_name: {
+                            "$ifNull": [
+                                {
+                                    "$getField": {
+                                        "input": {"$arrayElemAt": ["$group", 0]},
+                                        "field": "tmp_name",
+                                    }
+                                },
+                                [],
+                            ]
+                        }
+                    }
+                },
+            ]
+        )
+        # Erase all the projection given that we already project the required value.
+        subquery.project_fields = None
     compiler.subqueries.append(subquery)
     return f"${table_output}.{field_name}"