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activemodel.gemspec	activemodel.gemspec

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']}

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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"]

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Model name introspection

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

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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}"

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Internationalization (i18n) support

class Person extend ActiveModel::Translation end Person.human_attribute_name('my_attribute') # => "My attribute"

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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

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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