test ListCommentsForPost use case

backend/tests/Unit/Comment/UseCases/ListCommentsForPostTest.php

@@ -0,0 +1,73 @@
+<?php
+
+namespace Tests\Unit\Comment\UseCases;
+
+use App\Comment\CreateCommentDto;
+use App\Comment\UseCases\ListCommentsForPost\ListCommentsForPost;
+use App\Comment\UseCases\ListCommentsForPost\ListCommentsForPostRequest;
+use App\Exceptions\BadRequestException;
+use DateTimeImmutable;
+use DateTimeZone;
+use Tests\Fakes\FakeCommentRepository;
+use Tests\TestCase;
+
+class ListCommentsForPostTest extends TestCase
+{
+    private FakeCommentRepository $commentRepo;
+
+    private ListCommentsForPost $useCase;
+
+    private DateTimeImmutable $now;
+
+    protected function setUp(): void
+    {
+        $this->now = new DateTimeImmutable(
+            '2026-05-06T12:00:00',
+            new DateTimeZone('UTC'),
+        );
+        $this->commentRepo = new FakeCommentRepository;
+        $this->useCase = new ListCommentsForPost($this->commentRepo);
+    }
+
+    private function seed(int $postId, string $body, string $offset): void
+    {
+        $this->commentRepo->create(new CreateCommentDto(
+            postId: $postId,
+            userId: 1,
+            body: $body,
+            createdAt: $this->now->modify($offset),
+        ));
+    }
+
+    public function test_zero_post_id_throws_bad_request(): void
+    {
+        $this->expectException(BadRequestException::class);
+        $this->useCase->execute(new ListCommentsForPostRequest(
+            postId: 0,
+        ));
+    }
+
+    public function test_returns_empty_list_when_no_comments(): void
+    {
+        $result = $this->useCase->execute(new ListCommentsForPostRequest(
+            postId: 1,
+        ));
+
+        $this->assertSame([], $result);
+    }
+
+    public function test_returns_only_comments_for_given_post(): void
+    {
+        $this->seed(1, 'first', '+0 seconds');
+        $this->seed(2, 'other-post', '+0 seconds');
+        $this->seed(1, 'second', '+1 minute');
+
+        $result = $this->useCase->execute(new ListCommentsForPostRequest(
+            postId: 1,
+        ));
+
+        $this->assertCount(2, $result);
+        $this->assertSame('first', $result[0]->getBody());
+        $this->assertSame('second', $result[1]->getBody());
+    }
+}