Autopatch does not work with nullable fields

[betaprior]

Consider a simple struct with two nullable string fields:

type Foo struct {
	A *string `json:"a"`
	B *string `json:"b"`
}

It is evidently impossible to set these fields to nil via a PATCH with application/merge-patch+json content type. For instance, the following request will throw a 422 with a message "expected required property a to be present".

curl --request PATCH \
  --url http://localhost:8888/foo/1 \
  --header 'Accept: application/json, application/problem+json' \
  --header 'Content-Type: application/merge-patch+json' \
  --data '{
  "a": null
}'

This seems like a bug in merge patch computation: the PUT body in the autogenerated GET/PUT sequence omits the a field altogether instead of setting a: null as expected.

The only way to set a to nil appears to be via a PUT of the full struct or via a json-patch (not merge-patch):

curl --request PATCH \
  --url http://localhost:8888/foo/1 \
  --header 'Accept: application/json, application/problem+json' \
  --header 'Content-Type: application/json-patch+json' \
  --data '[
  { "op": "replace", "path": "/a", "value": null }
]

This came as an unfortunate surprise as our front-end client started making use of PATCH requests.