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-Active Record models, for example. Active Model also helps with building custom ORMs for use outside of the Rails framework.

You can read more about Active Model in the Active Model Basics guide.

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 by defining an explicit API. You can read more about the API in ActiveModel::Lint::Tests.

Active Model provides a default module that implements the basic API required to integrate with Action Pack out of the box: ::ActiveModel::API.

class Person
  include ActiveModel::API

  attr_accessor :name, :age
  validates_presence_of :name
end

person = Person.new(name: 'bob', age: '18')
person.name   # => 'bob'
person.age    # => '18'
person.valid? # => true

It includes model name introspections, conversions, translations and validations, resulting in a class suitable to be used with Action Pack. See ActiveModel::API for more examples.

Active Model also provides the following functionality to have ORM-like behavior out of the box:

• 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 = Person.new
  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

  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
  end
  
  person = Person.new
  person.name             # => nil
  person.changed?         # => false
  person.name = 'bob'
  person.changed?         # => true
  person.changed          # => ['name']
  person.changes          # => { 'name' => [nil, 'bob'] }
  person.save
  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, "cannot be nil") if name.nil?
    end
  
    def self.human_attribute_name(attr, options = {})
      "Name"
    end
  end
  
  person = Person.new
  person.name = nil
  person.validate!
  person.errors.full_messages
  # => ["Name cannot be nil"]

  Learn more

• Model name introspection

  class NamedPerson
    extend ActiveModel::Naming
  end
  
  NamedPerson.model_name.name   # => "NamedPerson"
  NamedPerson.model_name.human  # => "Named person"

  Learn more

• Making objects serializable

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

  class SerialPerson
    include ActiveModel::Serialization
  
    attr_accessor :name
  
    def attributes
      {'name' => name}
    end
  end
  
  s = SerialPerson.new
  s.serializable_hash   # => {"name"=>nil}
  
  class SerialPerson
    include ActiveModel::Serializers::JSON
  end
  
  s = SerialPerson.new
  s.to_json             # => "{\"name\":null}"

  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.start_with?("z")
    end
  end
  
  person = Person.new
  person.first_name = 'zoolander'
  person.valid?  # => false

  Learn more

• Custom validators

  class HasNameValidator < ActiveModel::Validator
    def validate(record)
      record.errors.add(:name, "must exist") if record.name.blank?
    end
  end
  
  class ValidatorPerson
    include ActiveModel::Validations
    validates_with HasNameValidator
    attr_accessor :name
  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:

$ gem install activemodel

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

• github.com/rails/rails/tree/main/activemodel

License

Active Model is released under the MIT license:

• opensource.org/licenses/MIT

Support

API documentation is at:

• api.rubyonrails.org

Bug reports for the Ruby on Rails project can be filed here:

• github.com/rails/rails/issues

Feature requests should be discussed on the rails-core mailing list here:

• discuss.rubyonrails.org/c/rubyonrails-core