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Active Model Basics

This guide should provide you with all you need to get started using model classes. Active Model allows for Action Pack helpers to interact with plain Ruby objects. Active Model also helps build custom ORMs for use outside of the Rails framework.

After reading this guide, you will know:

1 Introduction

Active Model is a library containing various modules used in developing classes that need some features present on Active Record. Some of these modules are explained below.

1.1 Attribute Methods

The ActiveModel::AttributeMethods module can add custom prefixes and suffixes on methods of a class. It is used by defining the prefixes and suffixes and 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

    def reset_attribute(attribute)
      send("#{attribute}=", 0)

    def attribute_highest?(attribute)
      send(attribute) > 100

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

1.2 Callbacks

ActiveModel::Callbacks gives Active Record style callbacks. This provides an ability to define callbacks which run at appropriate times. After defining callbacks, you can wrap them with before, after and around custom methods.

class Person
  extend ActiveModel::Callbacks

  define_model_callbacks :update

  before_update :reset_me

  def update
    run_callbacks(:update) do
      # This method is called when update is called on an object.

  def reset_me
    # This method is called when update is called on an object as a before_update callback is defined.

1.3 Conversion

If a class defines persisted? and id methods, then you can include the ActiveModel::Conversion module in that class, and call the Rails conversion methods on objects of that class.

class Person
  include ActiveModel::Conversion

  def persisted?

  def id

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

1.4 Dirty

An object becomes dirty when it has gone through one or more changes to its attributes and has not been saved. ActiveModel::Dirty gives the ability to check whether an object has been changed or not. It also has attribute based accessor methods. Let's consider a Person class with attributes first_name and last_name:

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

  def first_name

  def first_name=(value)
    @first_name = value

  def last_name

  def last_name=(value)
    @last_name = value

  def save
    # do save work...

1.4.1 Querying object directly for its list of all changed attributes.
person = Person.new
person.changed? # => false

person.first_name = "First Name"
person.first_name # => "First Name"

# returns true if any of the attributes have unsaved changes.
person.changed? # => true

# returns a list of attributes that have changed before saving.
person.changed # => ["first_name"]

# returns a Hash of the attributes that have changed with their original values.
person.changed_attributes # => {"first_name"=>nil}

# returns a Hash of changes, with the attribute names as the keys, and the
# values as an array of the old and new values for that field.
person.changes # => {"first_name"=>[nil, "First Name"]}

1.4.2 Attribute based accessor methods

Track whether the particular attribute has been changed or not.

# attr_name_changed?
person.first_name # => "First Name"
person.first_name_changed? # => true

Track the previous value of the attribute.

# attr_name_was accessor
person.first_name_was # => nil

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

# attr_name_change
person.first_name_change # => [nil, "First Name"]
person.last_name_change # => nil

1.5 Validations

The ActiveModel::Validations module adds the ability to validate objects like in Active Record.

class Person
  include ActiveModel::Validations

  attr_accessor :name, :email, :token

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

person = Person.new
person.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

1.6 Naming

ActiveModel::Naming adds a number of class methods which make naming and routing easier to manage. The module defines the model_name class method which will define a number of accessors using some ActiveSupport::Inflector methods.

class Person
  extend ActiveModel::Naming

Person.model_name.name                # => "Person"
Person.model_name.singular            # => "person"
Person.model_name.plural              # => "people"
Person.model_name.element             # => "person"
Person.model_name.human               # => "Person"
Person.model_name.collection          # => "people"
Person.model_name.param_key           # => "person"
Person.model_name.i18n_key            # => :person
Person.model_name.route_key           # => "people"
Person.model_name.singular_route_key  # => "person"

1.7 Model

ActiveModel::Model adds the ability for a class to work with Action Pack and Action View right out of the box.

class EmailContact
  include ActiveModel::Model

  attr_accessor :name, :email, :message
  validates :name, :email, :message, presence: true

  def deliver
    if valid?
      # deliver email

When including ActiveModel::Model you get some features like:

  • model name introspection
  • conversions
  • translations
  • validations

It also gives you the ability to initialize an object with a hash of attributes, much like any Active Record object.

email_contact = EmailContact.new(name: 'David',
                                 email: 'david@example.com',
                                 message: 'Hello World')
email_contact.name       # => 'David'
email_contact.email      # => 'david@example.com'
email_contact.valid?     # => true
email_contact.persisted? # => false

Any class that includes ActiveModel::Model can be used with form_for, render and any other Action View helper methods, just like Active Record objects.

1.8 Serialization

ActiveModel::Serialization provides basic serialization for your object. You need to declare an attributes Hash which contains the attributes you want to serialize. Attributes must be strings, not symbols.

class Person
  include ActiveModel::Serialization

  attr_accessor :name

  def attributes
    {'name' => nil}

Now you can access a serialized Hash of your object using the serializable_hash method.

person = Person.new
person.serializable_hash   # => {"name"=>nil}
person.name = "Bob"
person.serializable_hash   # => {"name"=>"Bob"}

1.8.1 ActiveModel::Serializers

Active Model also provides the ActiveModel::Serializers::JSON module for JSON serializing / deserializing. This module automatically includes the previously discussed ActiveModel::Serialization module. ActiveModel::Serializers::JSON

To use ActiveModel::Serializers::JSON you only need to change the module you are including from ActiveModel::Serialization to ActiveModel::Serializers::JSON.

class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes
    {'name' => nil}

The as_json method, similar to serializable_hash, provides a Hash representing the model.

person = Person.new
person.as_json # => {"name"=>nil}
person.name = "Bob"
person.as_json # => {"name"=>"Bob"}

You can also define the attributes for a model from a JSON string. However, you need to define the attributes= method on your class:

class Person
  include ActiveModel::Serializers::JSON

  attr_accessor :name

  def attributes=(hash)
    hash.each do |key, value|
      send("#{key}=", value)

  def attributes
    {'name' => nil}

Now it is possible to create an instance of Person and set attributes using from_json.

json = { name: 'Bob' }.to_json
person = Person.new
person.from_json(json) # => #<Person:0x00000100c773f0 @name="Bob">
person.name            # => "Bob"

1.9 Translation

ActiveModel::Translation provides integration between your object and the Rails internationalization (i18n) framework.

class Person
  extend ActiveModel::Translation

With the human_attribute_name method, you can transform attribute names into a more human-readable format. The human-readable format is defined in your locale file(s).

  • config/locales/app.pt-BR.yml
          name: 'Nome'

Person.human_attribute_name('name') # => "Nome"

1.10 Lint Tests

ActiveModel::Lint::Tests allows you to test whether an object is compliant with the Active Model API.

  • app/models/person.rb

    class Person
      include ActiveModel::Model
  • test/models/person_test.rb

    require 'test_helper'
    class PersonTest < ActiveSupport::TestCase
      include ActiveModel::Lint::Tests
      setup do
        @model = Person.new
$ rails test

Run options: --seed 14596

# Running:


Finished in 0.024899s, 240.9735 runs/s, 1204.8677 assertions/s.

6 runs, 30 assertions, 0 failures, 0 errors, 0 skips

An object is not required to implement all APIs in order to work with Action Pack. This module only intends to provide guidance in case you want all features out of the box.

1.11 SecurePassword

ActiveModel::SecurePassword provides a way to securely store any password in an encrypted form. When you include this module, a has_secure_password class method is provided which defines a password accessor with certain validations on it.

1.11.1 Requirements

ActiveModel::SecurePassword depends on bcrypt, so include this gem in your Gemfile to use ActiveModel::SecurePassword correctly. In order to make this work, the model must have an accessor named password_digest. The has_secure_password will add the following validations on the password accessor:

  1. Password should be present.
  2. Password should be equal to its confirmation (provided password_confirmation is passed along).
  3. The maximum length of a password is 72 (required by bcrypt on which ActiveModel::SecurePassword depends)
1.11.2 Examples
class Person
  include ActiveModel::SecurePassword
  attr_accessor :password_digest

person = Person.new

# When password is blank.
person.valid? # => false

# When the confirmation doesn't match the password.
person.password = 'aditya'
person.password_confirmation = 'nomatch'
person.valid? # => false

# When the length of password exceeds 72.
person.password = person.password_confirmation = 'a' * 100
person.valid? # => false

# When only password is supplied with no password_confirmation.
person.password = 'aditya'
person.valid? # => true

# When all validations are passed.
person.password = person.password_confirmation = 'aditya'
person.valid? # => true


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