mv-options PATCH raises KeyError: 'feature' (500) on partial update

[sentry]

Sentry: FLAGSMITH-API-69Q (https://flagsmith.sentry.io/issues/7627331389/)

What happens

A PATCH to /api/v1/projects/{project_pk}/features/{feature_pk}/mv-options/{pk}/ that omits feature from the body raises an unhandled KeyError: 'feature' → HTTP 500.

Root cause

MultivariateFeatureOptionSerializer.validate() (api/features/multivariate/serializers.py:83) reads attrs["feature"]:

def validate(self, attrs):
    attrs = super().validate(attrs)
    total_sibling_percentage_allocation = (
        self._get_siblings(attrs["feature"]).aggregate(...)
    )

On partial_update, feature is only present in attrs if the client sent it, so any partial PATCH that omits it (e.g. updating only default_percentage_allocation, type, or key) 500s.

Reproduce

PATCH /api/v1/projects/<p>/features/<f>/mv-options/<o>/
{ "default_percentage_allocation": 40 }
→ 500, KeyError: 'feature'

Including "feature": <f> in the body avoids it.

Suggested fix

Fall back to the instance on partial updates:

feature = attrs.get("feature") or getattr(self.instance, "feature", None)

Notes

Surfaced via the Flagsmith CLI (feature variant update sends partial PATCHes). Worked around CLI-side by always sending feature; other clients doing partial PATCHes still hit this.

[khvn26]

I don't think we properly support partial PATCH requests anywhere in our API, and not sure if we should.