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:


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:


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. (26 Jul 2016) The race condition in the TCP stack, answer
  2. (25 Jul 2016) The race condition in the TCP stack
  3. (28 Apr 2015) What is the meaning of this change?
  4. (26 Sep 2013) Spot the bug
  5. (27 May 2013) The problem of locking down tasks…
  6. (17 Oct 2011) Minimum number of round trips
  7. (23 Aug 2011) Recent Comments with Future Posts
  8. (02 Aug 2011) Modifying execution approaches
  9. (29 Apr 2011) Stop the leaks
  10. (23 Dec 2010) This code should never hit production
  11. (17 Dec 2010) Your own ThreadLocal
  12. (03 Dec 2010) Querying relative information with RavenDB
  13. (29 Jun 2010) Find the bug
  14. (23 Jun 2010) Dynamically dynamic
  15. (28 Apr 2010) What killed the application?
  16. (19 Mar 2010) What does this code do?
  17. (04 Mar 2010) Robust enumeration over external code
  18. (16 Feb 2010) Premature optimization, and all of that…
  19. (12 Feb 2010) Efficient querying
  20. (10 Feb 2010) Find the resource leak
  21. (21 Oct 2009) Can you spot the bug?
  22. (18 Oct 2009) Why is this wrong?
  23. (17 Oct 2009) Write the check in comment
  24. (15 Sep 2009) NH Prof Exporting Reports
  25. (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
  26. (01 Sep 2009) Why isn’t select broken?
  27. (06 Aug 2009) Find the bug fixes
  28. (26 May 2009) Find the bug
  29. (14 May 2009) multi threaded test failure
  30. (11 May 2009) The regex that doesn’t match
  31. (24 Mar 2009) probability based selection
  32. (13 Mar 2009) C# Rewriting
  33. (18 Feb 2009) write a self extracting program
  34. (04 Sep 2008) Don't stop with the first DSL abstraction
  35. (02 Aug 2008) What is the problem?
  36. (28 Jul 2008) What does this code do?
  37. (26 Jul 2008) Find the bug fix
  38. (05 Jul 2008) Find the deadlock
  39. (03 Jul 2008) Find the bug
  40. (02 Jul 2008) What is wrong with this code
  41. (05 Jun 2008) why did the tests fail?
  42. (27 May 2008) Striving for better syntax
  43. (13 Apr 2008) calling generics without the generic type
  44. (12 Apr 2008) The directory tree
  45. (24 Mar 2008) Find the version
  46. (21 Jan 2008) Strongly typing weakly typed code
  47. (28 Jun 2007) Windsor Null Object Dependency Facility