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

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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