Fix attribute_before_type_cast for serialized attributes. Fixes #4837.

activerecord/lib/active_record/attribute_methods/serialization.rb

@@ -88,6 +88,14 @@ module ActiveRecord
           super
         end
       end
+
+      def read_attribute_before_type_cast(attr_name)
+        if serialized_attributes.include?(attr_name)
+          super.unserialized_value
+        else
+          super
+        end
+      end
     end
   end
 end

activerecord/lib/active_record/core.rb

@@ -209,6 +209,8 @@ module ActiveRecord
     # The dup method does not preserve the timestamps (created|updated)_(at|on).
     def initialize_dup(other)
       cloned_attributes = other.clone_attributes(:read_attribute_before_type_cast)
+      self.class.initialize_attributes(cloned_attributes)
+
       cloned_attributes.delete(self.class.primary_key)
 
       @attributes = cloned_attributes

activerecord/test/cases/base_test.rb

@@ -1279,6 +1279,21 @@ class BasicsTest < ActiveRecord::TestCase
     assert_equal(hash, important_topic.content)
   end
 
+  # This test was added to fix GH #4004. Obviously the value returned
+  # is not really the value 'before type cast' so we should maybe think
+  # about changing that in the future.
+  def test_serialized_attribute_before_type_cast_returns_unserialized_value
+    klass = Class.new(ActiveRecord::Base)
+    klass.table_name = "topics"
+    klass.serialize :content, Hash
+
+    t = klass.new(:content => { :foo => :bar })
+    assert_equal({ :foo => :bar }, t.content_before_type_cast)
+    t.save!
+    t.reload
+    assert_equal({ :foo => :bar }, t.content_before_type_cast)
+  end
+
   def test_serialized_attribute_declared_in_subclass
     hash = { 'important1' => 'value1', 'important2' => 'value2' }
     important_topic = ImportantTopic.create("important" => hash)