Deduplicate post view notifications

src/test/java/com/openisle/service/NotificationServiceTest.java

@@ -168,4 +168,29 @@ class NotificationServiceTest {
         verify(email).sendEmail("a@a.com", "有人回复了你", "https://ex.com/posts/1#comment-2");
         verify(push).sendNotification(eq(user), contains("/posts/1#comment-2"));
     }
+
+    @Test
+    void postViewedDeletesOldNotification() {
+        NotificationRepository nRepo = mock(NotificationRepository.class);
+        UserRepository uRepo = mock(UserRepository.class);
+        ReactionRepository rRepo = mock(ReactionRepository.class);
+        EmailSender email = mock(EmailSender.class);
+        PushNotificationService push = mock(PushNotificationService.class);
+        Executor executor = Runnable::run;
+        NotificationService service = new NotificationService(nRepo, uRepo, email, push, rRepo, executor);
+
+        User author = new User();
+        author.setId(1L);
+        User viewer = new User();
+        viewer.setId(2L);
+        Post post = new Post();
+        post.setId(3L);
+
+        when(nRepo.save(any(Notification.class))).thenAnswer(i -> i.getArgument(0));
+
+        service.createNotification(author, NotificationType.POST_VIEWED, post, null, null, viewer, null, null);
+
+        verify(nRepo).deleteByUserAndTypeAndPostAndFromUser(author, NotificationType.POST_VIEWED, post, viewer);
+        verify(nRepo).save(any(Notification.class));
+    }
 }