Active Model Dirty

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

The requirements to implement ActiveModel::Dirty are to:

  • 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

If you wish to also track previous changes on save or update, you need to add

  @previously_changed = changes

inside of your save or update method.

A minimal implementation could be:

  class Person

    include ActiveModel::Dirty

    define_attribute_methods [:name]

    def name

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

    def save
      @previously_changed = 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.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'] }

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 << 'y'
  person.name_change    # => ['Bill', 'Billy']
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