Challenge 2: Nest a Resource With whenLoaded() — Possible Solution
====================================================================

// app/Http/Resources/PublisherResource.php

namespace App\Http\Resources;

use Illuminate\Http\Request;
use Illuminate\Http\Resources\Json\JsonResource;

class PublisherResource extends JsonResource
{
    public function toArray(Request $request): array
    {
        return [
            'id' => $this->id,
            'name' => $this->name,
        ];
    }
}

// app/Http/Resources/BookResource.php — extended

namespace App\Http\Resources;

use Illuminate\Http\Request;
use Illuminate\Http\Resources\Json\JsonResource;

class BookResource extends JsonResource
{
    public function toArray(Request $request): array
    {
        return [
            'id' => $this->id,
            'title' => $this->title,
            'price' => $this->price,
            'publisher' => new PublisherResource($this->whenLoaded('publisher')),
        ];
    }
}

// --- Demonstrating the difference ---

// app/Http/Controllers/BookController.php

use App\Http\Resources\BookResource;
use App\Models\Book;

public function indexWithoutEagerLoad()
{
    return BookResource::collection(Book::all());
}

public function indexWithEagerLoad()
{
    return BookResource::collection(Book::with('publisher')->get());
}

RESULTING OUTPUT DIFFERENCE
-------------------------------
indexWithoutEagerLoad() — publisher relationship NOT loaded:
{
  "data": [
    { "id": 1, "title": "Dune", "price": "12.99" }
  ]
}
// Notice "publisher" is entirely ABSENT from the JSON — not null,
// just omitted, because whenLoaded('publisher') detected the
// relationship was never loaded and skipped including the key at all.

indexWithEagerLoad() — publisher relationship eager-loaded first:
{
  "data": [
    { "id": 1, "title": "Dune", "price": "12.99",
      "publisher": { "id": 3, "name": "Ace Books" } }
  ]
}
// "publisher" now appears, nested as a full PublisherResource object.

WHY THIS WORKS
--------------
- new PublisherResource($this->whenLoaded('publisher')) does two things
  at once: whenLoaded('publisher') either returns the loaded Publisher
  model (if with('publisher') ran upstream) or a special "missing value"
  marker (if it wasn't loaded) — wrapping that in PublisherResource means
  the whole key gets omitted from the final JSON when the value is
  missing, rather than crashing or emitting "publisher": null.
- Critically, whenLoaded() NEVER triggers a fresh database query itself
  — it only checks whether the relationship happens to already be loaded
  in memory. This is exactly what makes it safe against the N+1 trap:
  using $this->publisher directly (without whenLoaded) would trigger a
  real lazy-load query if the relationship wasn't eager loaded, silently
  reintroducing N+1 for every book in the collection.
- This gives API consumers of the SAME BookResource class two different
  (but both correct) response shapes depending entirely on what the
  controller chose to eager load — a deliberate, controllable trade-off
  between response payload size and query efficiency.