fix: prevent 500 error on partial update of multivariate options

api/features/multivariate/serializers.py

@@ -82,15 +82,21 @@ def validate_key(self, value: str | None) -> str | None:
 
     def validate(self, attrs):  # type: ignore[no-untyped-def]
         attrs = super().validate(attrs)
-        feature = attrs["feature"]
-        default_percentage_allocation = attrs["default_percentage_allocation"]
+
+        # Safely get feature and allocation, falling back to instance for PATCH requests
+        feature = attrs.get("feature", getattr(self.instance, "feature", None))
+        default_percentage_allocation = attrs.get(
+            "default_percentage_allocation",
+            getattr(self.instance, "default_percentage_allocation", 0),
+        )
 
         total_sibling_percentage_allocation = (
             self._get_siblings(feature).aggregate(
                 total_percentage_allocation=Sum("default_percentage_allocation")
             )["total_percentage_allocation"]
             or 0
         )
+
         total_percentage_allocation = (
             total_sibling_percentage_allocation + default_percentage_allocation
         )
@@ -136,10 +142,12 @@ def _validate_environment_allocations(
             )
 
     def _validate_key_is_unique(self, attrs: dict[str, typing.Any]) -> None:
-        key = attrs.get("key")
+        key = attrs.get("key", getattr(self.instance, "key", None))
         if key is None:
             return
-        if self._get_siblings(attrs["feature"]).filter(key=key).exists():
+
+        feature = attrs.get("feature", getattr(self.instance, "feature", None))
+        if self._get_siblings(feature).filter(key=key).exists():
             raise ValidationError(
                 {
                     "key": "Multivariate option with this key already exists for the feature."

api/tests/unit/features/multivariate/test_unit_multivariate_views.py

@@ -1,3 +1,4 @@
+import json
 import uuid
 
 import pytest
@@ -105,3 +106,33 @@ def test_list_mv_options__feature_in_other_project__returns_404(
 
     # Then
     assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_partial_update_multivariate_option__valid_data__returns_200_and_updates(
+    admin_client: APIClient,
+    project: Project,
+    multivariate_feature: Feature,
+) -> None:
+    # Given
+    mv_option = multivariate_feature.multivariate_options.first()
+
+    assert mv_option is not None
+
+    url = f"/api/v1/projects/{project.id}/features/{multivariate_feature.id}/mv-options/{mv_option.id}/"
+
+    new_key = "hero"
+    data = {"key": new_key}
+
+    initial_allocation = mv_option.default_percentage_allocation
+
+    # When
+    response = admin_client.patch(
+        url, data=json.dumps(data), content_type="application/json"
+    )
+
+    # Then
+    assert response.status_code == status.HTTP_200_OK
+
+    mv_option.refresh_from_db()
+    assert mv_option.key == new_key
+    assert mv_option.default_percentage_allocation == initial_allocation