Add tests for duration multiplication and division

When multiplying or dividing a duration by a scalar, it's tempting to
operate directly on the duration's value in seconds and recompute the
parts from the result. However this loses information, as there are
multiple combinations of parts that map to any given number of seconds
(e.g. `2.weeks` or `336.hours`). This is especially problematic when
dealing with durations on the scale of months or years, as converting an
exact number of seconds to one of those intervals and then using the
resulting duration to modify a date will give the wrong result.

1 files changed, 12 insertions, 0 deletions

diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 240ae3bde0..63934e2433 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -158,6 +158,18 @@ class DurationTest < ActiveSupport::TestCase
     assert_equal Date.civil(2017, 1, 3), Date.civil(2017, 1, 1) + 1.day * 2
   end
 
+  def test_date_added_with_multiplied_duration_larger_than_one_month
+    assert_equal Date.civil(2017, 2, 15), Date.civil(2017, 1, 1) + 1.day * 45
+  end
+
+  def test_date_added_with_divided_duration
+    assert_equal Date.civil(2017, 1, 3), Date.civil(2017, 1, 1) + 4.days / 2
+  end
+
+  def test_date_added_with_divided_duration_larger_than_one_month
+    assert_equal Date.civil(2017, 2, 15), Date.civil(2017, 1, 1) + 90.days / 2
+  end
+
   def test_plus_with_time
     assert_equal 1 + 1.second, 1.second + 1, "Duration + Numeric should == Numeric + Duration"
   end