Active Model – model interfaces for Rails

Active Model provides a known set of interfaces for usage in model classes. They allow for Action Pack helpers to interact with non-ActiveRecord models, for example. Active Model also helps building custom ORMs for use outside of the Rails framework.

Prior to Rails 3.0, if a plugin or gem developer wanted to have an object interact with Action Pack helpers, it was required to either copy chunks of code from Rails, or monkey patch entire helpers to make them handle objects that did not exactly conform to the Active Record interface. This would result in code duplication and fragile applications that broke on upgrades.

Active Model solves this. You can include functionality from the following modules:

  • Add attribute magic to objects

    class Person
  include ActiveModel::AttributeMethods

  attribute_method_prefix 'clear_'
  define_attribute_methods [:name, :age]

  attr_accessor :name, :age

  def clear_attribute(attr)
    send("#{attr}=", nil)
  end
end

person.clear_name
person.clear_age

    Learn more

  • Callbacks for certain operations

    class Person
  extend ActiveModel::Callbacks
  define_model_callbacks :create

  def create
    run_callbacks :create do
      # Your create action methods here
    end
  end
end

    This generates before_create, around_create and after_create class methods that wrap your create method.

    Learn more

  • Tracking value changes

    The ActiveModel::Dirty module allows for tracking attribute changes:

    person = Person.new
person.name # => nil
person.changed? # => false
person.name = 'bob'
person.changed? # => true
person.changed # => ['name']
person.changes # => { 'name' => [nil, 'bob'] }
person.name = 'robert'
person.save
person.previous_changes # => {'name' => ['bob, 'robert']}

    Learn more

  • Adding errors interface to objects

    Exposing error messages allows objects to interact with Action Pack helpers seamlessly.

    class Person

  def initialize
    @errors = ActiveModel::Errors.new(self)
  end

  attr_accessor :name
  attr_reader   :errors

  def validate!
    errors.add(:name, "can not be nil") if name.nil?
  end

  def ErrorsPerson.human_attribute_name(attr, options = {})
    "Name"
  end

end

person.errors.full_messages
# => ["Name can not be nil"]

person.errors.full_messages
# => ["Name can not be nil"]

    Learn more

  • Model name introspection

    class NamedPerson
  extend ActiveModel::Naming
end

NamedPerson.model_name        # => "NamedPerson"
NamedPerson.model_name.human  # => "Named person"

    Learn more

  • Observer support

    ActiveModel::Observers allows your object to implement the Observer pattern in a Rails App and take advantage of all the standard observer functions.

    Learn more

  • Making objects serializable

    ActiveModel::Serialization provides a standard interface for your object to provide to_json or to_xml serialization.

    s = SerialPerson.new
s.serializable_hash   # => {"name"=>nil}
s.to_json             # => "{\"name\":null}"
s.to_xml              # => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<serial-person...

    Learn more

  • Internationalization (i18n) support

    class Person
  extend ActiveModel::Translation
end

Person.human_attribute_name('my_attribute')
# => "My attribute"

    Learn more

  • Validation support

    class Person
  include ActiveModel::Validations

  attr_accessor :first_name, :last_name

  validates_each :first_name, :last_name do |record, attr, value|
    record.errors.add attr, 'starts with z.' if value.to_s[0] == ?z
  end
end

person = Person.new
person.first_name = 'zoolander'
person.valid?  # => false

    Learn more

  • Custom validators

    class Person
  include ActiveModel::Validations
  validates_with HasNameValidator
  attr_accessor :name
end

class HasNameValidator < ActiveModel::Validator
  def validate(record)
   record.errors[:name] = "must exist" if record.name.blank?
  end
end

p = ValidatorPerson.new
p.valid?                  # =>  false
p.errors.full_messages    # => ["Name must exist"]
p.name = "Bob"
p.valid?                  # =>  true

    Learn more

Download and installation

The latest version of Active Model can be installed with Rubygems:

% [sudo] gem install activemodel

Source code can be downloaded as part of the Rails project on GitHub

License

Active Model is released under the MIT license.

Support

API documentation is at

Bug reports and feature requests can be filed with the rest for the Ruby on Rails project here: