How does using String#between? with DateTime objects work in Rails? - ruby

Question

Using Ruby 2.3.2, Rails 5.0.0.1
In Ruby console
2.3.2 :001 > date_time_str = "2015-01-08 08:17:15 UTC"
=> "2015-01-08 08:17:15 UTC"
2.3.2 :002 > date_time_range_start = "2015-01-08 08:16:15 UTC"
=> "2015-01-08 08:16:15 UTC"
2.3.2 :003 > date_time_range_end = "2015-01-08 08:20:15 UTC"
=> "2015-01-08 08:20:15 UTC"
2.3.2 :008 > require 'date'
=> true
2.3.2 :009 > date_time_str.between?(DateTime.parse(date_time_range_start), DateTime.parse(date_time_range_end))
ArgumentError: comparison of String with DateTime failed
from (irb):9:in `between?'
from (irb):9
from /home/jignesh/.rvm/rubies/ruby-2.3.2/bin/irb:11:in `<main>'
I expected the above behaviour as I was attempting to compare a String instance with a DateTime instance. However when I attempt the same in rails console it returns expected result as shown below.
In Rails console
Loading development environment (Rails 5.0.0.1)
2.3.2 :001 > date_time_str = "2015-01-08 08:17:15 UTC"
=> "2015-01-08 08:17:15 UTC"
2.3.2 :002 > date_time_range_start = "2015-01-08 08:16:15 UTC"
=> "2015-01-08 08:16:15 UTC"
2.3.2 :003 > date_time_range_end = "2015-01-08 08:20:15 UTC"
=> "2015-01-08 08:20:15 UTC"
2.3.2 :004 > date_time_str.between?(DateTime.parse(date_time_range_start), DateTime.parse(date_time_range_end))
=> true
2.3.2 :005 > date_time_str_2 = "2015-01-08 08:21:15 UTC"
=> "2015-01-08 08:21:15 UTC"
2.3.2 :006 > date_time_str_2.between?(DateTime.parse(date_time_range_end), DateTime.parse(r_e))
=> false
Can anybody please help me understand what Rails does internally to make the above work?

Answer 1

String#<=> in ruby tries to compare itself to the other object. If the other object is incomparable (not a string), then the implementation for string specifically (c-source here) compares the other object to the receiving string.
In other words, "str" <=> obj is equivalent to
if obj.is_a?(String)
# compare bytes
elsif obj.respond_to?(:to_str)
obj = obj.to_str
# compare bytes
else
# a few safety checks
-1 * (obj <=> "str")
end
Now we have "str" <=> obj boiling down to obj <=> "str". So far this is just Ruby, no Rails. Since Datetime#<=> has no special handling for string, it returns nil and story ends here.
In Rails, they redefined Datetime#<=> and defined String#to_datetime