Add support for customizing fields

test/serializer_test.rb

@@ -814,4 +814,58 @@ class SerializerTest < ActiveModel::TestCase
       ]
     }, actual)
   end
+
+  def test_can_customize_attributes
+    serializer = Class.new(ActiveModel::Serializer) do
+      attributes :title, :body
+
+      def title
+        object.title.upcase
+      end
+    end
+
+    klass = Class.new do
+      def read_attribute_for_serialization(name)
+        { :title => "New post!", :body => "First post body" }[name]
+      end
+
+      def title
+        read_attribute_for_serialization(:title)
+      end
+
+      def body
+        read_attribute_for_serialization(:body)
+      end
+    end
+
+    object = klass.new
+
+    actual = serializer.new(object, :root => :post).as_json
+
+    assert_equal({
+      :post => {
+        :title => "NEW POST!",
+        :body => "First post body"
+      }
+    }, actual)
+  end
+
+  def test_can_customize_attributes_with_read_attributes
+    serializer = Class.new(ActiveModel::Serializer) do
+      attributes :title, :body
+
+      def read_attribute_for_serialization(name)
+        { :title => "New post!", :body => "First post body" }[name]
+      end
+    end
+
+    actual = serializer.new(Object.new, :root => :post).as_json
+
+    assert_equal({
+      :post => {
+        :title => "New post!",
+        :body => "First post body"
+      }
+    }, actual)
+  end
 end