3 files changed, 27 insertions, 0 deletions
diff --git a/activesupport/lib/active_support/core_ext/hash.rb b/activesupport/lib/active_support/core_ext/hash.rb
index 501483498d..fd1cda991e 100644
--- a/activesupport/lib/active_support/core_ext/hash.rb
+++ b/activesupport/lib/active_support/core_ext/hash.rb
@@ -1,5 +1,6 @@
 require 'active_support/core_ext/hash/conversions'
 require 'active_support/core_ext/hash/deep_merge'
+require 'active_support/core_ext/hash/deep_dup'
 require 'active_support/core_ext/hash/diff'
 require 'active_support/core_ext/hash/except'
 require 'active_support/core_ext/hash/indifferent_access'
diff --git a/activesupport/lib/active_support/core_ext/hash/deep_dup.rb b/activesupport/lib/active_support/core_ext/hash/deep_dup.rb
new file mode 100644
index 0000000000..447142605c
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/hash/deep_dup.rb
@@ -0,0 +1,11 @@
+class Hash
+  # Returns a deep copy of hash.
+  def deep_dup
+    duplicate = self.dup
+    duplicate.each_pair do |k,v|
+      tv = duplicate[k]
+      duplicate[k] = tv.is_a?(Hash) && v.is_a?(Hash) ? tv.deep_dup : v
+    end
+    duplicate
+  end
+end
diff --git a/activesupport/test/core_ext/hash_ext_test.rb b/activesupport/test/core_ext/hash_ext_test.rb
index 545fed2684..bfcfddad47 100644
--- a/activesupport/test/core_ext/hash_ext_test.rb
+++ b/activesupport/test/core_ext/hash_ext_test.rb
@@ -316,6 +316,21 @@ class HashExtTest < Test::Unit::TestCase
     assert_equal expected, hash_1
   end
 
+  def test_deep_dup
+    hash = { :a => { :b => 'b' } }
+    dup = hash.deep_dup
+    dup[:a][:c] = 'c'
+    assert_equal nil, hash[:a][:c]
+    assert_equal 'c', dup[:a][:c]
+  end
+
+  def test_deep_dup_initialize
+    zero_hash = Hash.new 0
+    hash = { :a => zero_hash }
+    dup = hash.deep_dup
+    assert_equal 0, dup[:a][44]
+  end
+
   def test_store_on_indifferent_access
     hash = HashWithIndifferentAccess.new
     hash.store(:test1, 1)