Fix handling of $-prefixed string values during save

Author: WaVEV

django_mongodb_backend/compiler.py

@@ -885,6 +885,8 @@ def execute_sql(self, result_type):
             prepared = field.get_db_prep_save(value, connection=self.connection)
             if hasattr(value, "as_mql"):
                 prepared = prepared.as_mql(self, self.connection, as_expr=True)
+            if is_direct_value(value):
+                prepared = {"$literal": prepared}
             values[field.column] = prepared
         try:
             criteria = self.build_query().match_mql

django_mongodb_backend/expressions/builtins.py

@@ -211,9 +211,9 @@ def when(self, compiler, connection):
 
 def value(self, compiler, connection, as_expr=False):  # noqa: ARG001
     value = self.value
-    if isinstance(value, (list, int)) and as_expr:
-        # Wrap lists & numbers in $literal to prevent ambiguity when Value
-        # appears in $project.
+    if isinstance(value, (list, int, str)) and as_expr:
+        # Wrap lists, numbers, & string in $literal to prevent ambiguity when Value
+        # appears in $project or $update.
         return {"$literal": value}
     if isinstance(value, Decimal):
         return Decimal128(value)

django_mongodb_backend/test.py

@@ -19,3 +19,18 @@ def assertAggregateQuery(self, query, expected_collection, expected_pipeline):
             eval(pipeline[:-1], {"SON": SON, "ObjectId": ObjectId, "Decimal128": Decimal128}, {}),  # noqa: S307
             expected_pipeline,
         )
+
+    def assertUpdateQuery(self, query, expected_collection, expected_pipeline, new_values):
+        """
+        Assert that the logged query is equal to:
+            db.{expected_collection}.aggregate({expected_pipeline})
+        """
+        prefix, pipeline = query.split("(", 1)
+        _, collection, operator = prefix.split(".")
+        self.assertEqual(operator, "update_many")
+        self.assertEqual(collection, expected_collection)
+        condition, values = eval(  # noqa: S307
+            pipeline[:-1], {"SON": SON, "ObjectId": ObjectId, "Decimal128": Decimal128}, {}
+        )
+        self.assertEqual(condition, expected_pipeline)
+        self.assertEqual(values, new_values)