Fix child instance resolution in ListSerializer

rest_framework/serializers.py

@@ -662,14 +662,40 @@ class ListSerializer(BaseSerializer):
             if model is not None:
                 pk_name = model._meta.pk.name
 
+            obj_id = None
             if pk_name:
-                obj_id = data.get(pk_name, data.get("pk", data.get("id")))
-                if obj_id is not None:
-                    for obj in self.instance:
-                        if hasattr(obj, pk_name) and getattr(obj, pk_name) == obj_id:
-                            child_instance = obj
+                for field_name, field in self.child.fields.items():
+                    if getattr(field, "source", None) == pk_name:
+                        obj_id = data.get(field_name)
+                        if obj_id is not None:
                             break
 
+                if obj_id is None:
+                    obj_id = data.get(pk_name) or data.get("pk") or data.get("id")
+
+            resolved_instance = None
+
+            if obj_id is not None and pk_name:
+                try:
+                    obj_id = model._meta.pk.to_python(obj_id)
+                except Exception:
+                    pass
+
+                if not hasattr(self, "_instance_index"):
+                    self._instance_index = {
+                        getattr(obj, pk_name): obj for obj in self.instance
+                    }
+
+                resolved_instance = self._instance_index.get(obj_id)
+
+            if resolved_instance is None:
+                if model is not None and self.context.get("allow_create", True):
+                    resolved_instance = model()
+                else:
+                    resolved_instance = child_instance
+
+            child_instance = resolved_instance
+
         self.child.instance = child_instance
         self.child.initial_data = data
         return self.child.run_validation(data)

tests/test_serializer_lists.py

@@ -860,7 +860,7 @@ class TestManyTrueValidationCheck:
 
         input_data = [
             {
-                "uuid": "t3308237e-18d8-4074-9d05-79cc0fdb5bb3",
+                "uuid": "c20f2f31-65a3-451f-ae7d-e939b7d9f84b",
                 "name": "bar",
             },
         ]