module ActiveModel::Dirty

Active Model Dirty

Provides a way to track changes in your object in the same way as Active Record does.

The requirements for implementing ActiveModel::Dirty are:

• include ActiveModel::Dirty in your object.

• Call define_attribute_methods passing each method you want to track.

• Call attr_name_will_change! before each change to the tracked attribute.

• Call changes_applied after the changes are persisted.

• Call reset_changes when you want to reset the changes information.

A minimal implementation could be:

class Person
  include ActiveModel::Dirty

  define_attribute_methods :name

  def name
    @name
  end

  def name=(val)
    name_will_change! unless val == @name
    @name = val
  end

  def save
    # do persistence work
    changes_applied
  end

  def reload!
    reset_changes
  end
end

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_changed?(from: "Uncle Bob", to: "Bob") # => 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

Reset the changes:

person.previous_changes # => {"name" => ["Uncle Bob", "Bill"]}
person.reload!
person.previous_changes # => {}

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"]}

If an attribute is modified in-place then make use of [attribute_name]_will_change! to mark that the attribute is changing. Otherwise ActiveModel can't track changes to in-place attributes.

person.name_will_change!
person.name_change    # => ["Bill", "Bill"]
person.name << 'y'
person.name_change    # => ["Bill", "Billy"]