Fix: Prevent KeyError on partial PATCH updates to mv-options

api/features/multivariate/serializers.py

@@ -80,7 +80,7 @@ def validate_key(self, value: str | None) -> str | None:
             raise serializers.ValidationError(RESERVED_VARIANT_KEY_MESSAGE)
         return value
 
-    def validate(self, attrs):  # type: ignore[no-untyped-def]
+def validate(self, attrs):  # type: ignore[no-untyped-def]
         attrs = super().validate(attrs)
         feature = attrs["feature"]
         default_percentage_allocation = attrs["default_percentage_allocation"]
@@ -136,16 +136,19 @@ def _validate_environment_allocations(
             )
 
     def _validate_key_is_unique(self, attrs: dict[str, typing.Any]) -> None:
+        key = attrs.get("key") or getattr(self.instance, "key", None)
         key = attrs.get("key")
         if key is None:
             return
-        if self._get_siblings(attrs["feature"]).filter(key=key).exists():
+
+        feature = attrs.get("feature") or 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."
                 }
             )
-
     def _get_siblings(self, feature: Feature):  # type: ignore[no-untyped-def]
         siblings = feature.multivariate_options.all()
         if self.instance:

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

@@ -105,3 +105,25 @@ def test_list_mv_options__feature_in_other_project__returns_404(
 
     # Then
     assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_multivariate_feature_option_create_omitted_allocation_fails_if_siblings_exist(
+    feature,
+):
+    # Given an existing option taking up 50%
+    MultivariateFeatureOption.objects.create(
+        feature=feature,
+        default_percentage_allocation=50,
+        type="unicode",
+        string_value="control",
+    )
+
+    # When we try to create a new one without specifying an allocation
+    # (Serializer defaults to 100, so 50 + 100 = 150 > 100)
+    serializer = MultivariateFeatureOptionSerializer(
+        data={"feature": feature.id, "type": "unicode", "string_value": "variant"}
+    )
+
+    # Then it should fail validation
+    assert serializer.is_valid() is False
+    assert "default_percentage_allocation" in serializer.errors