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::Model
.
class Person
include ActiveModel::Model
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::Model
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
-
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
andafter_create
class methods that wrap your create method. -
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']}
-
Adding
errors
interface to objectsExposing 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, = {}) "Name" end end person = Person.new person.name = nil person.validate! person.errors. # => ["Name cannot be nil"]
-
Model name introspection
class NamedPerson extend ActiveModel::Naming end NamedPerson.model_name.name # => "NamedPerson" NamedPerson.model_name.human # => "Named person"
-
Making objects serializable
ActiveModel::Serialization
provides a standard interface for your object to provideto_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}"
-
Internationalization (i18n) support
class Person extend ActiveModel::Translation end Person.human_attribute_name('my_attribute') # => "My attribute"
-
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
-
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. # => ["Name must exist"] p.name = "Bob" p.valid? # => true
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
License
Active Model is released under the MIT license:
Support
API documentation is at:
Bug reports for the Ruby on Rails project can be filed here:
Feature requests should be discussed on the rails-core mailing list here: