FIX: Link notification to first unread post (#12868)

* FIX: Link notification to first unread post

If a topic with a few posts was posted in a watched category or with a
watched tag, the created notification would always point to the last
post, instead of pointing to the first one.

The root cause is that the query that fetched the first unread post
uses 'TopicUser' records and those are not created by default for
user watching a category or tag. In this case, it should use the
'CategoryUser' or 'TagUser' records.

* DEV: Use named bind variables

spec/services/post_alerter_spec.rb

@@ -1101,6 +1101,26 @@ describe PostAlerter do
         }.to add_notification(staged_member, :posted)
           .and not_add_notification(staged_non_member, :posted)
       end
+
+      it "does not update existing unread notification" do
+        category = Fabricate(:category)
+        CategoryUser.set_notification_level_for_category(user, CategoryUser.notification_levels[:watching], category.id)
+        topic = Fabricate(:topic, category: category)
+
+        post = Fabricate(:post, topic: topic)
+        PostAlerter.post_created(post)
+        notification = Notification.last
+        expect(notification.topic_id).to eq(topic.id)
+        expect(notification.post_number).to eq(1)
+
+        post = Fabricate(:post, topic: topic)
+        PostAlerter.post_created(post)
+        notification = Notification.last
+        expect(notification.topic_id).to eq(topic.id)
+        expect(notification.post_number).to eq(1)
+        notification_data = JSON.parse(notification.data)
+        expect(notification_data["display_username"]).to eq(I18n.t("embed.replies", count: 2))
+      end
     end
   end
 
@@ -1118,6 +1138,26 @@ describe PostAlerter do
         end
         expect(events).to include(event_name: :before_create_notifications_for_users, params: [[user], post])
       end
+
+      it "does not update existing unread notification" do
+        tag = Fabricate(:tag)
+        TagUser.change(user.id, tag.id, TagUser.notification_levels[:watching])
+        topic = Fabricate(:topic, tags: [tag])
+
+        post = Fabricate(:post, topic: topic)
+        PostAlerter.post_created(post)
+        notification = Notification.last
+        expect(notification.topic_id).to eq(topic.id)
+        expect(notification.post_number).to eq(1)
+
+        post = Fabricate(:post, topic: topic)
+        PostAlerter.post_created(post)
+        notification = Notification.last
+        expect(notification.topic_id).to eq(topic.id)
+        expect(notification.post_number).to eq(1)
+        notification_data = JSON.parse(notification.data)
+        expect(notification_data["display_username"]).to eq(I18n.t("embed.replies", count: 2))
+      end
     end
 
     context "on change" do