ChallengeRecent Comments with Future Posts

time to read 3 min | 575 words

We were asked to implement comments RSS for this blog, and many people asked about the recent comments widget. That turned out to be quite a bit more complicated than it appeared on first try, and I thought it would make a great challenge.

On the face of it, it looks like a drop dead simple feature, right? Show me the last 5 comments in the blog.

The problem with that is that it ignores something that is very important to me, the notion of future posts. One of the major advantages of RaccoonBlog is that I am able to post stuff that would go on the queue (for example, this post would be scheduled for about a month from the date of writing it), but still share that post with other people. Moreover, those other people can also comment on the post. To make things interesting, it is quite common for me to re-schedule posts, moving them from one date to another.

Given that complication, let us try to define how we want the Recent Comments feature to behave with regards to future posts. The logic is fairly simple:

  • Until the post is public, do not show the post comments.
  • If a post had comments on it while it was a future post, when it becomes public, the comments that were already posted there, should also be included.

The last requirement is a bit tricky. Allow me to explain. It would be easier to understand with an example, which luckily I have:

image_thumb[2]

As you can see, this is a post that was created on the 1st of July, but was published on the 12th. Mike and me have commented on the post shortly after it was published (while it was still hidden from the general public). Then, after it was published, grega_g and Jonty have commented on that.

Now, let us assume that we query at 12 Jul, 08:55 AM, we will not get any comments from this post, but on 12 Jul, 09:01 AM, we should get both comments from this post. To make things more interesting, those should come after comments that were posted (chronologically) after them. Confusing, isn’t it? Again, let us go with a visual aid for explaining things.

In other words, let us say that we also have this as well:

image

Here is what we should see in the Recent Comments:

12 Jul, 2011 – 08:55 AM 12 Jul, 2011 – 09:05 AM
  1. 07/07/2011 07:42 PM – jdn
  2. 07/08/2011 07:34 PM – Matt Warren
  1. 07/03/2011 05:26 PM – Ayende Rahien
  2. 07/03/2011 05:07 PM - Mike Minutillo
  3. 07/07/2011 07:42 PM – jdn
  4. 07/08/2011 07:34 PM – Matt Warren

Note that the 1st and 2snd location on 9:05 are should sort after 3rd and 4th, but are sorted before them, because of the post publish date, which we also take into account.

Given all of that, and regardless of the actual technology that you use, how would you implement this feature?

More posts in "Challenge" series:

  1. (03 Jan 2020) Spot the bug in the stream–answer
  2. (15 Feb 2010) Where is the optimization?