Saving MV features sometimes result in 500 server error

[matthewelwell]

When the FE is updating the multivariate_feature_state_values (MVFSVs) associated with a given feature state, it seems
to be using the ID of the multivariate_feature_option (MVFO), not the ID of the MVFSV. I've included some sample
requests below taken from the network console.

Step 1. User creates a new feature with 3 variations of 20, 30 & 40% weightings respectively

The request for this is irrelevant.

Step 2. User closes and reopens the feature in a given environment

FE makes a GET request to /api/v1/environments/:id/featurestates/:id/ with the following response body:

{
  "id": 71367,
  "feature_state_value": "control",
  "multivariate_feature_state_values": [
    {
      "id": 2704,
      "multivariate_feature_option": 784,
      "percentage_allocation": 40.0
    },
    {
      "id": 2702,
      "multivariate_feature_option": 783,
      "percentage_allocation": 30.0
    },
    {
      "id": 2700,
      "multivariate_feature_option": 782,
      "percentage_allocation": 20.0
    }
  ],
  "enabled": false,
  "feature": 13404,
  "environment": 7477,
  "identity": null,
  "feature_segment": null
}

Note that the ids of the MVFSVs themselves are different to the ids of the MVFOs.

Step 3. User attempts to resave the feature (without changing anything)

The FE makes a PUT request to /api/v1/environments/:id/featurestates/:id/ with the following body:

{
  "id": 71367,
  "feature_state_value": "control",
  "multivariate_feature_state_values": [
    {
      "id": 784,
      "multivariate_feature_option": 784,
      "percentage_allocation": 40
    },
    {
      "id": 783,
      "multivariate_feature_option": 783,
      "percentage_allocation": 30
    },
    {
      "id": 782,
      "multivariate_feature_option": 782,
      "percentage_allocation": 20
    }
  ],
  "identity": null,
  "enabled": false,
  "feature": 13404,
  "environment": 7477,
  "feature_segment": null
}

Note that the ids are now that of the MVFOs not the ids that were returned for the MVFSVs.