Return related posts for a specified blog post

Add the initial code for specifying a blog post and returning related
posts. This includes adding a repository for related posts, and adding
it as a service within the `opdavies_blog` module.

References #3

web/modules/custom/blog/opdavies_blog.services.yml

@@ -19,3 +19,6 @@ services:
 
   Drupal\opdavies_blog\Service\PostPusher\PostPusher:
     alias: Drupal\opdavies_blog\Service\PostPusher\IftttPostPusher
+
+  Drupal\opdavies_blog\Repository\RelatedPostsRepository:
+    autowire: true

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

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Drupal\opdavies_blog\Repository;
+
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\opdavies_blog\Entity\Node\Post;
+use Illuminate\Support\Collection;
+
+final class RelatedPostsRepository {
+
+  private EntityStorageInterface $nodeStorage;
+
+  public function __construct(
+    EntityTypeManagerInterface $entityTypeManager
+  ) {
+    $this->nodeStorage = $entityTypeManager->getStorage('node');
+  }
+
+  public function getFor(Post $post): Collection {
+    $posts = $this->nodeStorage->loadByProperties([
+      'type' => 'post',
+    ]);
+
+    return (new Collection($posts))
+      ->filter(fn(Post $relatedPost) => $relatedPost->id() != $post->id())
+      ->values();
+  }
+
+}

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

@@ -0,0 +1,38 @@
+<?php
+
+// phpcs:disable Drupal.Commenting.DocComment, Drupal.NamingConventions.ValidFunctionName
+
+namespace Drupal\Tests\opdavies_blog\Kernel;
+
+use Drupal\opdavies_blog\Repository\RelatedPostsRepository;
+use Drupal\opdavies_blog_test\Factory\PostFactory;
+
+final class RelatedPostsTest extends PostTestBase {
+
+  private RelatedPostsRepository $relatedPostsRepository;
+
+  /** @test */
+  public function it_returns_related_posts(): void {
+    $postA = (new PostFactory())->setTitle('Post A')
+      ->withTags(['Drupal 8'])
+      ->create();
+    $postA->save();
+
+    $postB = (new PostFactory())->setTitle('Post B')
+      ->withTags(['Drupal 8'])
+      ->create();
+    $postB->save();
+
+    $relatedPosts = $this->relatedPostsRepository->getFor($postA);
+
+    $this->assertCount(1, $relatedPosts);
+    $this->assertSame('Post B', $relatedPosts->first()->label());
+  }
+
+  protected function setUp() {
+    parent::setUp();
+
+    $this->relatedPostsRepository = $this->container->get(RelatedPostsRepository::class);
+  }
+
+}