Challenge 3: Fix an N+1 Across Two Relationship Types — Possible Solution
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// BEFORE — N+1 for BOTH relationships

use App\Models\Book;

$books = Book::all();  // 1 query
foreach ($books as $book) {
    echo $book->title . "\n";
    echo $book->author->name . "\n";           // +1 query PER book (belongsTo)
    foreach ($book->tags as $tag) {             // +1 query PER book (belongsToMany)
        echo $tag->name . "\n";
    }
}
// Total for N books: 1 + N + N = 1 + 2N queries

// AFTER — fixed with a single with() call

$books = Book::with(['author', 'tags'])->get();

foreach ($books as $book) {
    echo $book->title . "\n";
    echo $book->author->name . "\n";           // already loaded — no extra query
    foreach ($book->tags as $tag) {              // already loaded — no extra query
        echo $tag->name . "\n";
    }
}
// Total, regardless of N: exactly 3 queries (books, authors, tags)

WHY THIS WORKS
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- Book::with(['author', 'tags'])->get() passes an ARRAY of relationship
  names to a single with() call — this is the "one mechanism, any
  relationship type" simplification the chapter highlighted: author is a
  belongsTo relationship and tags is a belongsToMany relationship, but
  both are eager-loaded through the exact same with() method, unlike
  Django where a belongsTo-equivalent (ForeignKey) needs
  select_related() while a belongsToMany-equivalent (ManyToMany) needs
  the separate prefetch_related() method.
- Under the hood, Eloquent still runs multiple queries — one for the
  books themselves, one that fetches all the relevant authors in a
  single batched query, and one that fetches all the relevant tags (via
  the pivot table) in another single batched query — but critically,
  each of those runs exactly ONCE regardless of how many books are in
  the collection, rather than once per book.
- The total query count (3, in this case) is the concrete signal that
  the N+1 problem has actually been solved: it no longer scales with the
  number of books returned, exactly the same verification approach used
  when this problem was fixed in Rails and Django earlier in this
  project.