Dates and Integers in Active Support

16 Mar 2014

A quick experiment

Active Support monkey-patches lots of very useful helper methods into some basic Ruby types, like the date handling helpers that get mixed into Integers. Open up a Rails console and try something like this:

> one_month = 1.month
 => 1 month
> one_month.class
 => Fixnum
> one_month.to_i
 => 2592000
> one_month.to_i / 3600 / 24
 => 30

…so it looks like the month method returns a Fixnum representing the number of seconds in a 30 day month. So far so good.

> thirty_days = 30.days
 => 30 days
> thirty_days.to_i
 => 2592000
> thirty_days.to_i / 3600 / 24
 => 30

Now try:

> first_of_march = Date.new(2014, 3, 1)
 => Sat, 01 Mar 2014
> first_of_march - 30.days
 => Thu, 30 Jan 2014

Subtract 30 days from the 01/03/2014 and you get 30/01/2014 which makes sense because there are only 28 days in February.

Now try:

> first_of_march - 1.month
 => Sat, 01 Feb 2014

OK so now this is confusing. Active Support is doing the right thing here logically, subtract a whole month from a given date and you expect to be on the same day of the previous month. But we know that 1.month is just a Fixnum representing 30 days so what is going on here?

When is an integer more than just an integer?

To understand what is happening here you can dig into the Rails source code. It’s not too hard to do once you understand the high-level structure (e.g. the extensions to core Ruby classes are mostly found in the Active Support gem).

So if look at the implementation of Integer#month it starts to become a little clearer:

def months
  ActiveSupport::Duration.new(self * 30.days, [[:months, self]])
end
alias :month :months

So it doesn’t really return a Fixnum after all. It’s really an ActiveSupport::Duration. Now this class stores a value attribute (the Fixnum) and overrides method_missing delegating everything it doesn’t implement to value which is why it reports its class as Fixnum

def method_missing(method, *args, &block) #:nodoc:
  value.send(method, *args, &block)
end

ActiveSupport::Duration also has a parts attribute that stores the [[:months, 1]] in our case so it contains the additionally information, over and above the value, needed to perform logically correct date arithmetic. This gets applied in the extensions to the Date class that deal with date arithmetic. Check the source if you want to follow the trail further.

So it just works.

Is this a good thing?

There is a lot of magic in Rails like this and it can be controversial. On the whole it does the right thing and it obeys the principle of least surprise. So most of the Rails magic is a good thing because its pretty well judged. Many equally ‘clever’ features that are perhaps not quite so generally applicable don’t appear in the framework. You’ll have to make your own mind up, if this is too much magic for you there is always Sinatra.