active_model_basics.textile

Active Model Basics

This guide should provide you with all you need to get started using model classes. Active Model allow for Action Pack helpers to interact with non-ActiveRecord models. Active Model also helps building custom ORMs for use outside of the Rails framework.

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WARNING. This Guide is based on Rails 3.0. Some of the code shown here will not work in earlier versions of Rails.

Introduction

Active Model is a library containing various modules used in developing frameworks that need to interact with the Rails Action Pack library. Active Model provides a known set of interfaces for usage in classes. Some of modules are explained below -

AttributeMethods

AttributeMethods module can add custom prefixes and suffixes on methods of a class. It is used by defining the prefixes and suffixes, which methods on the object will use them.

class Person
include ActiveModel::AttributeMethods

attribute_method_prefix ‘reset_’ attribute_method_suffix ‘_highest?’ define_attribute_methods [‘age’] attr_accessor :age

private
def reset_attribute(attribute)
send(“#{attribute}=”, 0)
end

def attribute_highest?(attribute) send(attribute) > 100 ? true : false end

end

person = Person.new
person.age = 110
person.age_highest? # true
person.reset_age # 0
person.age_highest? # false

Callbacks

Callbacks gives Active Record style callbacks. This provides the ability to define the callbacks and those will run at appropriate time. After defining a callbacks you can wrap with before, after and around custom methods.

class Person
extend ActiveModel::Callbacks

define_model_callbacks :update before_update :reset_me def update _run_update_callbacks do

1. This will call when we are trying to call update on object.
   end
   end

def reset_me

1. This method will call when you are calling update on object as a before_update callback as defined.
   end
   end
   

Conversion

If a class defines persisted? and id methods then you can include Conversion module in that class and you can able to call Rails conversion methods to objects of that class.

class Person
include ActiveModel::Conversion

def persisted? false end def id nil end

end

person = Person.new
person.to_model == person #=> true
person.to_key #=> nil
person.to_param #=> nil

Dirty

An object becomes dirty when an object is gone through one or more changes to its attributes and not yet saved. This gives the ability to check whether an object has been changed or not. It also has attribute based accessor methods. Lets consider a Person class with attributes first_name and last_name

require ‘rubygems’
require ‘active_model’

class Person
include ActiveModel::Dirty
define_attribute_methods [:first_name, :last_name]

def first_name @first_name end def first_name=(value) first_name_will_change! @first_name = value end def last_name @last_name end def last_name=(value) last_name_will_change! @last_name = value end def save @previously_changed = changes end

end

Querying object directly for its list of all changed attributes.

person = Person.new
person.first_name = “First Name”

person.first_name #=> “First Name”
person.first_name = “First Name Changed”

person.changed? #=> true

#returns an list of fields arry which all has been changed before saved.
person.changed #=> [“first_name”]

#returns a hash of the fields that have changed with their original values.
person.changed_attributes #=> {"first_name" => “First Name Changed”}

#returns a hash of changes, with the attribute names as the keys, and the values will be an array of the old and new value for that field.
person.changes #=> {"first_name" => [“First Name”,“First Name Changed”]}

Attribute based accessor methods

Track whether the particular attribute has been changed or not.

#attr_name_changed?
person.first_name #=> “First Name”

#assign some other value to first_name attribute
person.first_name = “First Name 1”

person.first_name_changed? #=> true

Track what was the previous value of the attribute.

#attr_name_was accessor
person.first_name_was #=> “First Name”

Track both previous and current value of the changed attribute. Returns an array if changed else returns nil

#attr_name_change
person.first_name_change #=> [“First Name”, “First Name 1”]
person.last_name_change #=> nil

Validations

Validations module adds the ability to class objects to validate them in Active Record style.

class Person
include ActiveModel::Validations

attr_accessor :name, :email, :token validates :name, :presence => true validates_format_of :email, :with => /^([^\s]+)((?:[-a-z0-9]\.)[a-z]{2,})$/i validates! :token, :presence => true

end

person = Person.new(:token => “2b1f325”)
person.valid? #=> false
person.name = ‘vishnu’
person.email = ‘me’
person.valid? #=> false
person.email = ‘me@vishnuatrai.com’
person.valid? #=> true
person.token = nil
person.valid? #=> raises ActiveModel::StrictValidationFailed