4 files changed, 57 insertions, 0 deletions
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index f1dd7c312d..478fef2baf 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Add `flatten` and `flatten!` methods to Duration objects.
+
+    Example:
+         Date.today + (1.month + 1.month).flatten == Date.today + 2.months
+
+    *Ionatan Wiznia*
+
 *   `require_dependency` accepts objects that respond to `to_path`, in
     particular `Pathname` instances.
 
diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index 87b6407038..1db0ca51ae 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -81,6 +81,18 @@ module ActiveSupport
       to_i
     end
 
+    # Flattens all the +parts+ of the duration, and returns a
+    # new duration object.
+    def flatten
+      Duration.new(@value, flatten_parts)
+    end
+
+    # Flattens all the +parts+ of this duration.
+    def flatten!
+      @parts = flatten_parts
+      self
+    end
+
     protected
 
       def sum(sign, time = ::Time.current) #:nodoc:
@@ -97,6 +109,14 @@ module ActiveSupport
         end
       end
 
+      def flatten_parts
+        @parts.inject({}) do |result, (name, value)|
+          result[name] ||= 0
+          result[name] += value
+          result
+        end.to_a
+      end
+
     private
 
       def method_missing(method, *args, &block) #:nodoc:
diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index ed267cf4b9..cc24d7c74c 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -145,6 +145,21 @@ class DurationTest < ActiveSupport::TestCase
     assert_equal '172800', 2.days.to_json
   end
 
+  def test_flatten
+    a = 2.months
+    b = (1.month + 1.month).flatten
+
+    assert_equal a.parts, b.parts
+  end
+
+  def test_flatten!
+    a = (1.month + 1.month)
+    b = a.flatten
+    a.flatten!
+
+    assert_equal a.parts, b.parts
+  end
+
   protected
     def with_env_tz(new_tz = 'US/Eastern')
       old_tz, ENV['TZ'] = ENV['TZ'], new_tz
diff --git a/guides/source/active_support_core_extensions.md b/guides/source/active_support_core_extensions.md
index d3f49b19fa..d11c17117e 100644
--- a/guides/source/active_support_core_extensions.md
+++ b/guides/source/active_support_core_extensions.md
@@ -3699,6 +3699,21 @@ Time.utc(1582, 10, 3) + 5.days
 # => Mon Oct 18 00:00:00 UTC 1582
 ```
 
+When addinng or substracting durations, the resulting duration will be equivalent to subsequent calls to since or advance, so:
+
+```ruby
+Date.new(2013,1,28) + 1.month + 1.month
+# => Thu, 28 Mar 2013
+```
+
+If you want to add durations and then use them as just one call to since or advance, you can use the flatten or flatten! method:
+
+```ruby
+Date.new(2013,1,31) + (1.month + 1.month).flatten
+# => Sun, 31 Mar 2013
+```
+
+
 Extensions to `File`
 --------------------