Wrong behavior of ActiveModel::Errors#dup is causing regressions on applications using Rails 3-2-stable

activemodel/lib/active_model/errors.rb

@@ -79,6 +79,19 @@ def initialize(base)
       @messages = ActiveSupport::OrderedHash.new
     end
 
+    def initialize_dup(other)
+      @messages = other.messages.dup
+    end
+
+    # Backport dup from 1.9 so that #initialize_dup gets called
+    unless Object.respond_to?(:initialize_dup)
+      def dup # :nodoc:
+        copy = super
+        copy.initialize_dup(self)
+        copy
+      end
+    end
+
     # Clear the messages
     def clear
       messages.clear
@@ -119,7 +132,7 @@ def [](attribute)
     #   p.errors[:name] = "must be set"
     #   p.errors[:name] # => ['must be set']
     def []=(attribute, error)
-      self[attribute.to_sym] << error
+      self[attribute] << error
     end
 
     # Iterates through each error key, value pair in the error messages hash.

activemodel/test/cases/errors_test.rb

@@ -46,6 +46,14 @@ def test_has_key?
     assert errors.has_key?(:foo), 'errors should have key :foo'
   end
 
+  def test_dup
+    errors = ActiveModel::Errors.new(self)
+    errors[:foo] = 'bar'
+    errors_dup = errors.dup
+    errors_dup[:bar] = 'omg'
+    assert_not_same errors_dup.messages, errors.messages
+  end
+
   test "should return true if no errors" do
     person = Person.new
     person.errors[:foo]