January 16th, 2011
Comparing times and dates in Ruby
In one of the Rails projects I’m involved with, we do most of the local development against SQLite and then deploy against Oracle. This is a bit annoying for many reasons, but by far the largest cause of trouble is the handling of dates. I haven’t exactly figured out the rules, but for some reason sometimes Oracle returns DateTime in situations where SQLite returns a Date. This usually causes quite subtle problems that have effects in other parts of the application. This brings me to the small piece of advice I wanted to talk about in this column. Always make sure that you know if you are working with a Date, a Time or a DateTime, since these all have slightly different behavior, especially when it comes to comparisons.
The rule is quite simple. If you think you can have a Time object, make sure to turn it into a DateTime object before trying to compare it to a Date object. What happens otherwise? Unfunny things:
Date.today < Time.now #ArgumentError: comparison of Date with Time failed Time.now > Date.today #true Time.now == Date.today #false Date.today == Time.now #nil Date.today <=> Time.now #nil Date.today != Time.now #true
The first time I saw some of these results, I was a bit confused. Especially the last three. But they do make a twisted kind of sense. Namely, it’s OK for the <=> operator to return nil if it can’t do a comparison between two objects. And the != in Ruby is hardcoded to return the inverse of the value returned from ==, and the since nil is a falsey value, the inverse of that becomes true.
What I wanted to mention with these things is that you should always make sure you don’t have the Date on the left hand side of a comparison. Or if you want to do a comparison, explicitly call to_date to coerce them. Finally, if you want to do date and time comparisons, I find the best behavior usually comes from coercing both sides with to_datetime before doing the comparison.
