Correct null relationship in JSON API

Question

I have a JSON API type with a relationship

relationships {
    fruit: {data: {id: 123, type: "fruits"}}
}

What do I do if an instance does not have a fruit relationship?

Should I do this

relationships {
    fruit: {data: {id: null, type: "fruits"}}
}

or this

relationships {
    fruit: {data: null}
}

or this

relationships {
    fruit: null
}

or something completely different?

Answer 1

Accordingly to the spec a resource linkage object may have these values:

Resource linkage MUST be represented as one of the following:

• null for empty to-one relationships.
• an empty array ([]) for empty to-many relationships.
• a single resource identifier object for non-empty to-one relationships.
• an array of resource identifier objects for non-empty to-many relationships.

https://jsonapi.org/format/#document-resource-object-linkage

From your examples only

"relationships": {
    "fruit": {
      "data": null
    }
}

is a valid relationship object accordingly to JSON:API specification.

This relationship object tells your client that

• the resource object has a relationship fruit,
• that it's a to-one relationship,
• and that there isn't any related resource.