It turns out this is very easy to do. The latest versions of Git have an amend command. Amend lets us alter the last commit we made. All that’s necessary is for us to arrange our working directory the way we want the last commit to look and then execute:

git commit --amend

This will update the most recent commit based on the state of our working directory. For example, say we changed our README file in the last commit and accidentally introduced a typo. To fix the last commit, we would edit the README again, git add the change, and instead of running git commit which would create a second commit, we run git commit --amend which patches the last commit. This can be repeated as many times as necessary.

Note that rewriting history like this can have serious implications if you’ve already published the most recent commit. But if you’re the only developer using the repository, or if you haven’t published yet, this can be a great way to fix minor mistakes without introducing an entirely new commit.

You can read more about amend in the documentation.

For example, say I have a table of records with created_at timestamps. My view displays all records with timestamps equal to or before the current time. Let’s assume the current time is 15:00:00 precisely and I happen to have a record with a timestamp of 15:00:00 in the database. The SQL comparison would work fine in this case.

SELECT * FROM records WHERE created_at <= "2010-06-25 15:00:00"
=> 1 row in set

What if the timestamp in the database is 15:00:03 though? Let’s run the query again.

SELECT * FROM records WHERE created_at <= "2010-06-25 15:00:00"
=> Empty set

Since 15:00:03 is greater than the current time of 15:00:00, the record doesn’t get returned. This would be fine if we were displaying seconds in the view, but we’re not. From the user’s perspective, the timestamp on the record is still 15:00 and should appear in the view since it’s equal to the current time. But it doesn’t.

One way to fix this would be to handle the time comparisons in Ruby. This is certainly a legitimate option. For this particular project, though, performance was a big issue. (And we all know that Rails can’t scale.) I needed a way to continue letting the database handle the comparisons while disregarding seconds.

The solution I ended up with isn’t ideal (it relies on a couple of functions built into MySQL) but it works fine and runs fast:

SELECT * FROM records WHERE (created_at - INTERVAL SECOND(created_at) SECOND) <= "2010-06-25 15:00:00"
=> 1 row in set

The number of seconds is extracted from the created_at timestamp and then subtracted from the timestamp. So if the timestamp was 15:00:03, MySQL subtracts 3 seconds to end up with 15:00:00.

This solved the comparison problem for me and made my client very happy. Double win.