Return an empty Collection if there are no posts

Return an empty Collection if there are no related posts for the given
post.

References #3

web/modules/custom/blog/src/Repository/RelatedPostsRepository.php

@@ -7,6 +7,7 @@ namespace Drupal\opdavies_blog\Repository;
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\opdavies_blog\Entity\Node\Post;
+use Drupal\taxonomy\TermInterface;
 use Illuminate\Support\Collection;
 
 final class RelatedPostsRepository {
@@ -20,11 +21,23 @@ final class RelatedPostsRepository {
   }
 
   public function getFor(Post $post): Collection {
+    $tags = $post->get('field_tags')->referencedEntities();
+
+    if (!$tags) {
+      return new Collection();
+    }
+
+    $tagIds = (new Collection($tags))
+      ->map(fn(TermInterface $tag) => $tag->id())
+      ->values();
+
     $query = $this->nodeStorage->getQuery();
 
     // Ensure that the current node ID is not returned as a related post.
     $query->condition('nid', $post->id(), '!=');
 
+    $query->condition('field_tags', $tagIds->toArray(), 'IN');
+
     /** @var array $postIds */
     $postIds = $query->execute();
 

web/modules/custom/blog/tests/src/Kernel/RelatedPostsTest.php

@@ -33,6 +33,25 @@ final class RelatedPostsTest extends PostTestBase {
     $this->assertSame('Post B', $relatedPosts->first()->label());
   }
 
+  /** @test */
+  public function it_returns_an_empty_collection_if_there_are_no_related_posts(): void {
+    $postA = $this->postFactory
+      ->setTitle('Drupal 8 post')
+      ->withTags(['Drupal 8'])
+      ->create();
+    $postA->save();
+
+    $postB = $this->postFactory
+      ->setTitle('Drupal 9 post')
+      ->withTags(['Drupal 9'])
+      ->create();
+    $postB->save();
+
+    $relatedPosts = $this->relatedPostsRepository->getFor($postA);
+
+    $this->assertEmpty($relatedPosts);
+  }
+
   protected function setUp() {
     parent::setUp();