Refactor in-place dirty checking to use the attribute object

activerecord/lib/active_record/attribute.rb

@@ -29,6 +29,14 @@ def value_for_database
       type.type_cast_for_database(value)
     end
 
+    def changed_from?(old_value)
+      type.changed?(old_value, value, value_before_type_cast)
+    end
+
+    def changed_in_place_from?(old_value)
+      type.changed_in_place?(old_value, value)
+    end
+
     def type_cast
       raise NotImplementedError
     end
@@ -52,5 +60,16 @@ def type_cast(value)
         type.type_cast_from_user(value)
       end
     end
+
+    class Null
+      class << self
+        attr_reader :value, :value_before_type_cast, :value_for_database
+
+        def changed_from?(*)
+          false
+        end
+        alias changed_in_place_from? changed_from?
+      end
+    end
   end
 end

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -137,9 +137,7 @@ def keys_for_partial_write
       end
 
       def _field_changed?(attr, old_value)
-        new_value = read_attribute(attr)
-        raw_value = read_attribute_before_type_cast(attr)
-        column_for_attribute(attr).changed?(old_value, new_value, raw_value)
+        attribute_named(attr).changed_from?(old_value)
       end
 
       def changed_in_place
@@ -149,10 +147,8 @@ def changed_in_place
       end
 
       def changed_in_place?(attr_name)
-        type = type_for_attribute(attr_name)
         old_value = original_raw_attribute(attr_name)
-        value = read_attribute(attr_name)
-        type.changed_in_place?(old_value, value)
+        attribute_named(attr_name).changed_in_place_from?(old_value)
       end
 
       def original_raw_attribute(attr_name)
@@ -166,9 +162,7 @@ def original_raw_attributes
       end
 
       def store_original_raw_attribute(attr_name)
-        type = type_for_attribute(attr_name)
-        value = type.type_cast_for_database(read_attribute(attr_name))
-        original_raw_attributes[attr_name] = value
+        original_raw_attributes[attr_name] = attribute_named(attr_name).value_for_database
       end
 
       def store_original_raw_attributes

activerecord/lib/active_record/attribute_methods/read.rb

@@ -99,6 +99,10 @@ def read_attribute(attr_name)
       def attribute(attribute_name)
         read_attribute(attribute_name)
       end
+
+      def attribute_named(attribute_name)
+        @attributes.fetch(attribute_name, Attribute::Null)
+      end
     end
   end
 end