Mark notifications as read in a single SQL query

There is no need to load all comments only to count them. Lets just let
the database do all the work. If there are no comments found, nothing
will happen anyway.

Also already filter the comments to only search for notifications for
own comments.

And add some tests :)

app/services/post_service.rb

@@ -104,9 +104,8 @@ class PostService
   end
 
   def mark_like_on_comment_notifications_read(post_id)
-    comment_ids = Comment.where(commentable_id: post_id)
-
-    Notification.where(recipient_id: user.id, target_type: "Comment", target_id: comment_ids, unread: true)
-                .update_all(unread: false) if comment_ids.any?
+    Notification.where(recipient_id: user.id, target_type: "Comment",
+                       target_id: Comment.where(commentable_id: post_id, author_id: user.person.id),
+                       unread: true).update_all(unread: false) # rubocop:disable Rails/SkipsModelValidations
   end
 end

spec/services/post_service_spec.rb

@@ -168,6 +168,50 @@ describe PostService do
       expect_any_instance_of(PostService).not_to receive(:mark_mention_notifications_read).with(post.id)
       PostService.new.mark_user_notifications(post.id)
     end
+
+    context "for comments" do
+      let(:comment) { post.comments.create(author: alice.person, text: "comment") }
+
+      it "marks a corresponding notifications as read" do
+        FactoryBot.create(:notification, recipient: alice, target: comment, unread: true)
+        FactoryBot.create(:notification, recipient: alice, target: comment, unread: true)
+
+        expect {
+          PostService.new(alice).mark_user_notifications(post.id)
+        }.to change(Notification.where(unread: true), :count).by(-2)
+      end
+
+      it "does not change the update_at date/time for comment notifications" do
+        notification = Timecop.travel(1.minute.ago) do
+          FactoryBot.create(:notification, recipient: alice, target: comment, unread: true)
+        end
+
+        expect {
+          PostService.new(alice).mark_user_notifications(post.id)
+        }.not_to(change { Notification.where(id: notification.id).pluck(:updated_at) })
+      end
+
+      it "does not change other users notifications" do
+        alice_notification = FactoryBot.create(:notification, recipient: alice, target: comment, unread: true)
+        bob_notification = FactoryBot.create(:notification, recipient: bob, target: comment, unread: true)
+
+        PostService.new(alice).mark_user_notifications(post.id)
+
+        expect(Notification.find(alice_notification.id).unread).to be_falsey
+        expect(Notification.find(bob_notification.id).unread).to be_truthy
+      end
+
+      it "marks notifications for all comments on a post as read" do
+        comment2 = post.comments.create(author: alice.person, text: "comment2")
+
+        FactoryBot.create(:notification, recipient: alice, target: comment, unread: true)
+        FactoryBot.create(:notification, recipient: alice, target: comment2, unread: true)
+
+        expect {
+          PostService.new(alice).mark_user_notifications(post.id)
+        }.to change(Notification.where(unread: true), :count).by(-2)
+      end
+    end
   end
 
   describe "#destroy" do