Filter by subjects.

app/controllers/App.php

@@ -141,7 +141,7 @@ class App extends Controller
                 $book['date-published'] = (isset($book['metadata']['datePublished'])) ?
                     $book['metadata']['datePublished'] :
                     '';
-                $book['subject'] = (isset($book['metadata']['keywords'])) ? $book['metadata']['keywords'] : '';
+                $book['subject'] = (isset($book['metadata']['about'][0])) ? $book['metadata']['about'][0]['identifier'] : '';
                 $books[] = $book;
             }
             if ($orderby === 'latest') {

resources/views/partials/book.blade.php

@@ -1,11 +1,11 @@
 <div class="book"
   data-date-published="{{ str_replace('-', '', @$book['metadata']['datePublished']) }}"
   data-license="{{ (new \Pressbooks\Licensing())->getLicenseFromUrl($book['metadata']['license']['url']) }}"
-  data-subject="{{ sanitize_title(@$book['metadata']['keywords']) }}"
+  data-subject="{{ substr(@$book['subject'], 0, 2) }}"
 >
   @if(isset($book['metadata']['keywords']))
   <p class="book__subject">
-    <a href="{{ network_home_url('/catalog/#') . $book['metadata']['keywords'] }}">{{ $book['metadata']['keywords'] }}</a>
+    <a href="{{ network_home_url('/catalog/#') . substr($book['subject'], 0, 2) }}">{{ \Pressbooks\Metadata\get_subject_from_thema($book['subject']) }}</a>
   </p>
   @endif
   <p class="book__title">