1 files changed, 70 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/attribute_mutation_tracker.rb b/activerecord/lib/active_record/attribute_mutation_tracker.rb
new file mode 100644
index 0000000000..0133b4d0be
--- /dev/null
+++ b/activerecord/lib/active_record/attribute_mutation_tracker.rb
@@ -0,0 +1,70 @@
+module ActiveRecord
+  class AttributeMutationTracker # :nodoc:
+    def initialize(attributes)
+      @attributes = attributes
+    end
+
+    def changed_values
+      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
+        if changed?(attr_name)
+          result[attr_name] = attributes[attr_name].original_value
+        end
+      end
+    end
+
+    def changes
+      attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
+        if changed?(attr_name)
+          result[attr_name] = [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
+        end
+      end
+    end
+
+    def changed?(attr_name)
+      attr_name = attr_name.to_s
+      attributes[attr_name].changed?
+    end
+
+    def changed_in_place?(attr_name)
+      attributes[attr_name].changed_in_place?
+    end
+
+    def forget_change(attr_name)
+      attr_name = attr_name.to_s
+      attributes[attr_name] = attributes[attr_name].forgetting_assignment
+    end
+
+    protected
+
+    attr_reader :attributes
+
+    private
+
+    def attr_names
+      attributes.keys
+    end
+  end
+
+  class NullMutationTracker # :nodoc:
+    include Singleton
+
+    def changed_values
+      {}
+    end
+
+    def changes
+      {}
+    end
+
+    def changed?(*)
+      false
+    end
+
+    def changed_in_place?(*)
+      false
+    end
+
+    def forget_change(*)
+    end
+  end
+end