Fallback to fetching the association if relationship data was not found in included

[kamal]

Given a payload that looks something like

{
  "data": {
    ...
    "relationships": {
      "owner": {
        "links": { ... },
        "data": { "type": "owners", "id": 1 }
      }
    }
  }
}

where the relationship has data, but whose full data does not appear in included, the current code will raise an exception undefined method '[]' for nil:NilClass.

This PR changes how the included data is accessed that avoids this exception, by using Hash#dig. It then falls back to fetching the relationship via a request.

[senid231]

this is looks like a bug on server side if relationship has data corresponding object must be included in payload @kamal do you have real life case with such behaviour?

[abuiles]

@senid231 by reading the spec and based on interactions with other clients (ember-data) --- it doesn't seem like you must include a resource in the payload if you are using a resource linkage in the relationships data.

From https://jsonapi.org/format/#document-compound-documents

Resource linkage in a compound document allows a client to link together all of the included resource objects without having to GET any URLs via links.

The behavior I've seen is: if there is a linkage and the resources in included then user it, otherwise GET it.

However, if you include a resource in included, then it must have a linkage:

In a compound document, all included resources MUST be represented as an array of resource objects in a top-level included member -

[senid231]

@abuiles I get your point. Indeed I can't find in spec requirement that object must be present in included or dataset if it's id and type are present in relationships. I've always used it as de facto rule