Compound responses

Something like json-api may return JSON with many objects in it:

{
  "links": {
    "books.author": {
      "href": "http://example.com/authors/{books.author}",
      "type": "authors"
    }
  },
  "books": [{
    "id": "1",
    "title": "When In Doubt, Roll",
    "links": {
      "author": "1"
    }
  }],
  "linked": {
    "authors": [{
      "id": "1",
      "name": "Bill Bruford"
    }]
  }
}

Normally we would fetch an Author from http://example.com/people/{id}. However, if we already got this JSON for a book, we can reuse it.

Let’s write a Source for an Author, that will be able to use both AuthorEndpoint and CompoundBookEndpoint.

The first step is to understand that Source can take 4 parameters:

1. Endpoints used by default
2. How to get data from each supported endpoint
3. Priority for supported endpoints
4. How to add more optimal endpoints

We only need the first three to make it work.

The default endpoint is AuthorEndpoint:

def needAuthor(id: String) =
  Source[Author](AuthorEndpoint(id))(
    ...,
    ...
  )

Next, we show how to load an Author from both endpoints:

{
  case e @ AuthorEndpoint(`id`) ⇒
    // just load normally
    Resolvable[A].fromEndpoint(e)
  case e @ CompoundBookEndpoint(_) ⇒
    // inside the data returned by the endpoint,
    // try to find the author we are looking for
    // if not found, the endpoint will be simply skipped
    Resolvable[A].fromEndpointPath(e) { js ⇒
      (js \ "linked" \ "authors").find(_ \ "id" == JsString(id)).get
    }
}

Finally, we need to assign higher priority to the compound endpoint, to avoid unnecessary fetching of AuthorEndpoint:

{
  case CompoundBookEndpoint(_) ⇒ Seq(1)
  case AuthorEndpoint(_) ⇒ Seq(0) // default
}

Assembling everything together, we get:

def needAuthor(id: String) =
  Source[Author](AuthorEndpoint(id))({
    case e @ AuthorEndpoint(`id`) ⇒
      Resolvable[A].fromEndpoint(e)
    case e @ CompoundBookEndpoint(_) ⇒
      Resolvable[A].fromEndpointPath(e) { js ⇒
        (js \ "linked" \ "authors").find(_ \ "id" == JsString(id)).get
      }
  }, {
    case CompoundBookEndpoint(_) ⇒ Seq(1)
    case AuthorEndpoint(_) ⇒ Seq(0) // default
  })