Part 1 of many to come

“Convention over configuration”. So the saying goes.

While often applied to Rails’ ability to create a lot of functionality with relatively spares amounts of code, I also take a liking to this saying as synonymous with the principle of least surpise: code design should pack as few surprises as possible while accomplishing its goals. You take advantage of this philosophy every time you assume to_s returns a reasonable String representation of any object, or that .nil? returns a Boolean value according to whether or not ab object is nil. Technically, a codeing ne’er-do-well could easily write the nil? class to return a random number. In Ruby, they could just as eaily overwrite the nil? method on Object, which would of course screw up a lot.

I’ve already hinted at how Rails disobeys this principle more than I’m confortable with. However I recently came upon a bug-inducing example that really took be aback. So now I’m going to write about it.

As a convenience, I had written a lower_bound, upper_bound, and bound method into the Numeric class in an application – I could just no longer abide using using the ‘min’ method to mean upped bound and max for lower bound. “But wait,” you say, “isn’t that doing exactly the objectification that you just said in that post that you hate doing?” I mean, yeah, a little, but this is for convenience in a specific application, not for a gem or framework that will be distributed and imported in other applications. It’s just easier and more fun to write x.upper_bound(3.0) than something like But no, I would never be so presumptuous in a gem for distribution, thanks for asking.

Anyway, to the point. I wanted to expand this method to DateTime objects as well. So where to put it? Our app uses time zones extensively, so ActiveSupport::TimeWithZone as well as plain old Time need this method. Luckily:
# => true

Wonderful! Just put it on Time and we’re set! I do so with a core_ext/time.rb file. But then I notice something funny:
# => Thu, 30 Mar 2017 18:08:29 EDT -04:00
# => Thu, 30 Mar 2017 18:08:30 EDT -04:00
# => Thu, 30 Mar 2017 17:38:32 EDT -04:00

Huh? Lower-bounding by a day ago worked as expected (no change), but lower-bounding by half an hour ago returns half an hour ago? I could have probed further to discover that the 4-hour time zone shift was the cutoff for this weird behavior, but this time I went straight for the pry:

2.3.3 :001 >

From: /Users/andrew/3p/app3/lib/boundable.rb @ line 7 Boundable#lower_bound:

     6: def lower_bound(bound)
 =>  7:   binding.hpry
     8:   if self < bound
     9:     respond_to?(:coerce) ? coerce(bound).first : bound
    10:   else
    11:     self
    12:   end
    13: end

# => 2017-03-30 18:46:45 UTC
# => Thu, 30 Mar 2017 18:16:45 EDT -04:00
self < bound
# => true
# => Time
# => ActiveSupport::TimeWithZone

To summarize, “now” is apparently less than “30 minutes ago”. More precisely, the object returned by is evaluating as less than 30.minutes.ago, because when evaluating this line of code, self is a regular Time object (in UTC), and is being compared against a ActiveSupport::TimeWithZone object, but as a regular Time object it doesn’t know about time zones.

But why did self forget about its true identity as a TimeWithZone? Let’s sanity check how we’re getting into this method:
# => ActiveSupport::TimeWithZone
# => nil
# => ["/Users/andrew/3p/app3/lib/boundable.rb", 6]

So the owner is ActiveSupport::TimeWithZone, but there’s not source location? Except there is for a regular Time object? We need more basic sanity checking:
 => Object
 => false

So ActiveSupport::TimeWithZone ingerits directly from Object, not from the Time class in any way. We already cofirmed that; this appears to contradict that finding. Let’s get to the stack trace using caller in the pry. Highlighting just the relevant portion, we have:

"/Users/andrew/3p/app3/lib/boundable.rb:7:in `lower_bound'",
"/Users/andrew/.rvm/gems/ruby-2.3.3/gems/activesupport-4.2.3/lib/active_support/time_with_zone.rb:371:in `method_missing'",

Ah, the dreaded “method_missing” pattern. I have found few valid uses of method_missing, and this, in ActiveSupport’s very own time_with_zone.rb, is NOT one of them:

    # Send the missing method to +time+ instance, and wrap result in a new
    # TimeWithZone with the existing +time_zone+.
    def method_missing(sym, *args, &block)
      wrap_with_time_zone time.__send__(sym, *args, &block)
    rescue NoMethodError => e
      raise e, e.message.sub(time.inspect, self.inspect), e.backtrace

Here, time, of class Time, is an instance variable belonging to TimeWithZone, and this is the object that gave us the bug by not being able to correclty compare itself to bound, above. This method_missing pattern appears to evaluate the unknown method on the zone-normalized local Time object, because the authors thought that would basically be good enough. In a way it’s impementing a poor-man’s inheritance scheme. It works for the most part, but causes slippery bugs in edge cases such as ours: the < method returned the wrong result against a TimeWithZone object. Surprise!

More generally, if sym here is a method that calls any other method overwritten by TimeWithZone, this will not work correctly. Turns out, as much as Rubyists diss inheritance, method_missing is not actually a good substitute for it. Who’d have thought?

Edge cases are fine to overlook in an application as a business decision, but the more general and distributed your code gets the more important it should be to have all edge cases make some logical sense. This case does not. In a package as widespread as Rails, decisions like these are just sloppy.

For our use case here, I just slapped the same methods on ActiveSupport::TimeWithZone as well, despire them already being included in Time, which we’re supposed to get for free except for the fact that someone thought method_missing was an effective substitute for inheritance.

I wasn’t done digging: we still have this hanging fact that Turns out:
# => ["/Users/andrew/.rvm/gems/ruby-2.3.3/gems/activesupport-4.2.3/lib/active_support/time_with_zone.rb", 334]

You overwrote is_a?. You freakin overwrote is_a?.

    # Say we're a Time to thwart type checking.
    def is_a?(klass)
      klass == ::Time || super


That cuts deep, Rails. Too bad I just can’t quit you…