Add DateTime#subsec

Mirrors the Time#subsec method by returning the fraction
of the second as a Rational.

activesupport/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add `DateTime#subsec` to return the fraction of a second as a `Rational`.
+
+    *Andrew White*
+
 *   Add additional aliases for `DateTime#utc` to mirror the ones on
     `ActiveSupport::TimeWithZone` and `Time`.
 

activesupport/lib/active_support/core_ext/date_time/calculations.rb

@@ -28,6 +28,13 @@ def seconds_until_end_of_day
     end_of_day.to_i - to_i
   end
 
+  # Returns the fraction of a second as a +Rational+
+  #
+  #   DateTime.new(2012, 8, 29, 0, 0, 0.5).subsec # => (1/2)
+  def subsec
+    sec_fraction
+  end
+
   # Returns a new DateTime where one or more of the elements have been changed
   # according to the +options+ parameter. The time options (<tt>:hour</tt>,
   # <tt>:min</tt>, <tt>:sec</tt>) reset cascadingly, so if only the hour is

activesupport/test/core_ext/date_time_ext_test.rb

@@ -400,4 +400,9 @@ def test_nsec
     assert_equal 0, DateTime.civil(2000).nsec
     assert_equal 500000000, DateTime.civil(2000, 1, 1, 0, 0, Rational(1,2)).nsec
   end
+
+  def test_subsec
+    assert_equal 0, DateTime.civil(2000).subsec
+    assert_equal Rational(1,2), DateTime.civil(2000, 1, 1, 0, 0, Rational(1,2)).subsec
+  end
 end