1 files changed, 112 insertions, 0 deletions
diff --git a/activemodel/lib/active_model/dirty.rb b/activemodel/lib/active_model/dirty.rb
new file mode 100644
index 0000000000..624c3647ca
--- /dev/null
+++ b/activemodel/lib/active_model/dirty.rb
@@ -0,0 +1,112 @@
+module ActiveModel
+  # Track unsaved attribute changes.
+  #
+  # A newly instantiated object is unchanged:
+  #   person = Person.find_by_name('Uncle Bob')
+  #   person.changed?       # => false
+  #
+  # Change the name:
+  #   person.name = 'Bob'
+  #   person.changed?       # => true
+  #   person.name_changed?  # => true
+  #   person.name_was       # => 'Uncle Bob'
+  #   person.name_change    # => ['Uncle Bob', 'Bob']
+  #   person.name = 'Bill'
+  #   person.name_change    # => ['Uncle Bob', 'Bill']
+  #
+  # Save the changes:
+  #   person.save
+  #   person.changed?       # => false
+  #   person.name_changed?  # => false
+  #
+  # Assigning the same value leaves the attribute unchanged:
+  #   person.name = 'Bill'
+  #   person.name_changed?  # => false
+  #   person.name_change    # => nil
+  #
+  # Which attributes have changed?
+  #   person.name = 'Bob'
+  #   person.changed        # => ['name']
+  #   person.changes        # => { 'name' => ['Bill', 'Bob'] }
+  #
+  # Resetting an attribute returns it to its original state:
+  #   person.reset_name!    # => 'Bill'
+  #   person.changed?       # => false
+  #   person.name_changed?  # => false
+  #   person.name           # => 'Bill'
+  #
+  # Before modifying an attribute in-place:
+  #   person.name_will_change!
+  #   person.name << 'y'
+  #   person.name_change    # => ['Bill', 'Billy']
+  module Dirty
+    extend ActiveSupport::Concern
+    include ActiveModel::AttributeMethods
+
+    included do
+      attribute_method_suffix '_changed?', '_change', '_will_change!', '_was'
+      attribute_method_affix :prefix => 'reset_', :suffix => '!'
+    end
+
+    # Do any attributes have unsaved changes?
+    #   person.changed? # => false
+    #   person.name = 'bob'
+    #   person.changed? # => true
+    def changed?
+      !changed_attributes.empty?
+    end
+
+    # List of attributes with unsaved changes.
+    #   person.changed # => []
+    #   person.name = 'bob'
+    #   person.changed # => ['name']
+    def changed
+      changed_attributes.keys
+    end
+
+    # Map of changed attrs => [original value, new value].
+    #   person.changes # => {}
+    #   person.name = 'bob'
+    #   person.changes # => { 'name' => ['bill', 'bob'] }
+    def changes
+      changed.inject({}) { |h, attr| h[attr] = attribute_change(attr); h }
+    end
+
+    private
+      # Map of change <tt>attr => original value</tt>.
+      def changed_attributes
+        @changed_attributes ||= {}
+      end
+
+      # Handle <tt>*_changed?</tt> for +method_missing+.
+      def attribute_changed?(attr)
+        changed_attributes.include?(attr)
+      end
+
+      # Handle <tt>*_change</tt> for +method_missing+.
+      def attribute_change(attr)
+        [changed_attributes[attr], __send__(attr)] if attribute_changed?(attr)
+      end
+
+      # Handle <tt>*_was</tt> for +method_missing+.
+      def attribute_was(attr)
+        attribute_changed?(attr) ? changed_attributes[attr] : __send__(attr)
+      end
+
+      # Handle <tt>*_will_change!</tt> for +method_missing+.
+      def attribute_will_change!(attr)
+        begin
+          value = __send__(attr)
+          value = value.duplicable? ? value.clone : value
+        rescue TypeError, NoMethodError
+        end
+
+        changed_attributes[attr] = value
+      end
+
+      # Handle <tt>reset_*!</tt> for +method_missing+.
+      def reset_attribute!(attr)
+        __send__("#{attr}=", changed_attributes[attr]) if attribute_changed?(attr)
+      end
+  end
+end