= 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}[link:classes/ActiveModel/AttributeMethods.html]
* 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}[link:classes/ActiveModel/CallBacks.html]
* 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}[link:classes/ActiveModel/Dirty.html]
* 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}[link:classes/ActiveModel/Errors.html]
* Model name introspection
class NamedPerson
extend ActiveModel::Naming
end
NamedPerson.model_name # => "NamedPerson"
NamedPerson.model_name.human # => "Named person"
{Learn more}[link:classes/ActiveModel/Naming.html]
* 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}[link:classes/ActiveModel/Observer.html]
* 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}[link:classes/ActiveModel/Serialization.html]
* Internationalization (i18n) support
class Person
extend ActiveModel::Translation
end
Person.human_attribute_name('my_attribute')
# => "My attribute"
{Learn more}[link:classes/ActiveModel/Translation.html]
* 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}[link:classes/ActiveModel/Validations.html]
* 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}[link:classes/ActiveModel/Validator.html]
