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 exacly 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
  

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• Callbacks for certain operations

  class Person
    extend ActiveModel::Callbacks
    define_model_callbacks :create
  
    def create
      _run_create_callbacks 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.

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• 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']}
  

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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, "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"]
  

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• Model name introspection

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

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• 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.

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• 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...
  

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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.to_s[0] == ?z
    end
  end
  
  person = Person.new
  person.first_name = 'zoolander'
  person.valid?  # => false
  

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• 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
  

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