ActiveType Tests

Make any Ruby object quack like ActiveRecord

ActiveType is our take on "presenter models" (or "form models") in Rails. We want to have controllers (and forms) talk to models that are either not backed by a database table, or have additional functionality that should not be shared to the rest of the application.

However, we do not want to lose ActiveRecord's amenities, like validations, callbacks, etc.

Examples for use cases are models to support sign in:

class SignIn < ActiveType::Object

  # this is not backed by a db table

  attribute :username, :string
  attribute :password, :string

  validates :username, presence: true
  validates :password, presence: true

  # ...


Or models to support sign up:

class User < ActiveRecord::Base
  # ...

class SignUp < ActiveType::Record[User]

  # this inherits from User

  validates :password, confirmation: true

  after_create :send_confirmation_email

  def send_confirmation_email
    # this should happen on sign-up, but not when creating a user in tests etc.

  # ...


A note on Rails's own .attribute

Rails 5+ comes with its own implementation of .attribute. This implementation is functionally very similar, but not identical to ActiveType's.

We have decided to continue to use our own implementation. This means that if you use ActiveType, ActiveRecord::Base.attribute will be overriden.

The following behaviours are different than in vanilla Rails 5:

  • Defaults procs are evaluated in instance context, not class context.
  • Defaults are evaluated lazily.
  • You can override attributes with custom methods and use super.
  • Attributes will work on records retrieved via .find.
  • Attributes will be duped if you dup the record.
  • You cannot use attribute :db_column to override the behaviour of an existing database-backed attribute.

If you need to use ActiveRecord's own .attribute method, you can still access is as ar_attribute:

class User < ApplicationRecord
  # use my custom type to serialize to the database
  ar_attribute :password,


Inherit from ActiveType::Object if you want an ActiveRecord-kind class that is not backed by a database table.

You can define "columns" by saying attribute:

class SignIn < ActiveType::Object

  attribute :email, :string
  attribute :date_of_birth, :date
  attribute :accepted_terms, :boolean
  attribute :account_type


These attributes can be assigned via constructor, mass-assignment, and are automatically typecast:

 = "1980-01-01", accepted_terms: "1", account_type:
.date_of_birth.class # Date
.accepted_terms? # true

ActiveType knows all the types that are allowed in migrations (i.e. :string, :integer, :float, :decimal, :datetime, :time, :date, :boolean). You can also skip the type to have a virtual attribute without typecasting.

ActiveType::Object actually inherits from ActiveRecord::Base, but simply skips all database access, inspired by ActiveRecord Tableless.

This means your object has all usual ActiveRecord::Base methods. Some of those might not work properly, however. What does work:

  • validations
  • callbacks (use before_save, after_save, not before_create, or before_update)
  • "saving" (returning true or false, without actually persisting)
  • belongs_to (after saying attribute :child_id, :integer)

Note on transactions

Since ActiveType::Object is not backed by a database, it does not open a real transaction when saving, so you should not rely on database changes that might have happend in a #save callback to be rolled back when #save fails. If you need this, make sure to wrap those changes in an explicit transaction yourself.


If you have a database backed record (that inherits from ActiveRecord::Base), but also want to declare virtual attributes, simply inherit from ActiveType::Record.

Virtual attributes will not be persisted.


ActiveType::Record[BaseClass] is used to extend a given BaseClass (that itself has to be an ActiveRecord model) with additional functionality, that is not meant to be shared to the rest of the application.

Your class will inherit from BaseClass. You can add additional methods, validations, callbacks, as well as use (virtual) attributes like an ActiveType::Object:

class SignUp < ActiveType::Record[User]
  # ...

If you need to access the extended BaseClass from your presenter model, you may call extended_record_base_class on its class:

SignUp.extended_record_base_class # => "User (...)"

# or
.class # => "SignUp (...)"
.class.extended_record_base_class # => "User (...)"

Inheriting from ActiveType:: objects

If you want to inherit from an ActiveType class, simply do

class SignUp < ActiveType::Record[User]
  # ...

class SpecialSignUp < SignUp
  # ...


Attributes can have defaults. Those are lazily evaluated on the first read, if no value has been set.

class SignIn < ActiveType::Object

  attribute :created_at, :datetime, default: proc { }


The proc is evaluated in the context of the object, so you can do

class SignIn < ActiveType::Object

  attribute :email, :string
  attribute :nickname, :string, default: proc { email.split('@').first }

end "").nickname # "tobias" "", :nickname => "kratob").nickname # "kratob"

Overriding accessors

You can override attribute getters and setters using super:

class SignIn < ActiveType::Object

  attribute :email, :string
  attribute :nickname, :string

  def email

  def nickname=(value)


Nested attributes

ActiveType supports its own variant of nested attributes via the nests_one / nests_many macros. The intention is to be mostly compatible with ActiveRecord's accepts_nested_attributes functionality.

Assume you have a list of records, say representing holidays, and you want to support bulk editing. Then you could do something like:

class Holiday < ActiveRecord::Base
  validates :date, presence: true

class HolidaysForm < ActiveType::Object
  nests_many :holidays, reject_if: :all_blank, default: proc { Holiday.all }

class HolidaysController < ApplicationController
  def edit
    @holidays_form =

  def update
    @holidays_form =[:holidays_form])
      redirect_to root_url, notice: "Success!"
      render :edit


# and in the view
<%= form_for @holidays_form, url: '/holidays', method: :put do |form| %>
    <%= form.fields_for :holidays do |holiday_form| %>
      <li><%= holiday_form.text_field :date %></li>
    <% end %>
<% end %>
  • You have to say nests_many :records
  • records will be validated and saved automatically
  • The generated .records_attributes = expects parameters like ActiveRecord's nested attributes, and works together with the fields_for helper:

    • either as a hash (where the keys are meaningless)
      '1' => { date: "new record's date" },
      '2' => { id: '3', date: "existing record's date" }
    • or as an array
      { date: "new record's date" },
      { id: '3', date: "existing record's date" }

To use it with single records, use nests_one. It works like accept_nested_attributes does for has_one. Use .record_attributes = to build the child record.

Supported options for nests_many / nests_one are:

  • build_scope

Used to build new records, for example:

  nests_many :documents, build_scope: proc { Document.where(:state => "fresh") }
  • find_scope

Used to find existing records (in order to update them).

  • scope

Sets find_scope and build_scope together.

If you don't supply a scope, ActiveType will guess from the association name, i.e. saying

  nests_many :documents

is the same as saying

  nests_many :documents, scope: proc { Document }

which is identical to

  nests_many :documents, build_scope: proc { Document }, find_scope: proc { Document }

All ...scope options are evaled in the context of the record on first use, and cached.

  • allow_destroy

Allow to destroy records if the attributes contain _destroy => '1'

  • reject_if

Pass either a proc of the form proc { |attributes| ... }, or a symbol indicating a method, or :all_blank.

Will reject attributes for which the proc or the method returns true, or with only blank values (for :all_blank).

  • default

Initializes the association on first access with the given proc:

  nests_many :documents, default: proc { Documents.all }

Options supported exclusively by nests_many are:

  • index_errors

Use a boolean to get indexed errors on related records. In Rails 5 you can make it global with config.active_record.index_nested_attribute_errors = true.

Casting records or relations

When working with ActiveType you will often find it useful to cast an ActiveRecord instance to its extended ActiveType::Record variant.

Use ActiveType.cast for this:

class User < ActiveRecord::Base

class SignUp < ActiveType::Record[User]

user = User.find(1)
sign_up = ActiveType.cast(user, SignUp)
sign_up.is_a?(SignUp) # => true

This is basically like ActiveRecord#becomes, but with less bugs and more consistent behavior.

Note that cast is destructive. The originally casted record (user) and the returned record (sign_up) share internal state (such as attributes). To avoid unexpected behavior, the original record will raise an error when trying to change or persist it. Also, casting of a record that has changes in its loaded associations is prevented, because those changes would be lost.
If you know what you are doing and absolutely want that, you may use the option force: true to allow this potentially problematic behaviour, e.g. sign_up = ActiveType.cast(user, SignUp, force: true)

You can also cast an entire relation (scope) to a relation of an ActiveType::Record:

adult_users = User.where('age >= 18')
adult_sign_ups = ActiveType.cast(adult_users, SignUp)
 = adult_sign_ups.find(1)
.is_a?(SignUp) # => true


Sometimes, you have an association, and a form model for that association. Instead of always casting the associations manually, you can use the change_association macro to override an association's options. For example.

class Credential < ActiveRecord::Base

class User < ActiveRecord::Base
  has_many :credentials

class SignUpCredential < ActiveType::Record[Credential]

class SignUp < ActiveType::Record[User]
  change_association :credentials, class_name: 'SignUpCredential'

Now, if you load credentials, you will automatically receive records of type SignUpCredential.

Supported Rails versions

ActiveType is tested against ActiveRecord 5.2, 6.0, 6.1, and 7.0.

Later versions might work, earlier will not.

Supported Ruby versions

ActiveType is tested against 2.5, 2.7, 3.0, and 3.1.


In your Gemfile say:

gem 'active_type'

Now run bundle install and restart your server.


  • We run tests against several ActiveRecord and Ruby versions using gemika.
  • You can bundle all versions with rake matrix:install.
  • You can run specs against all Gemfiles compatible with your current ruby version with rake matrix:spec.
  • You can run specs against a single Gemfile with BUNDLE_GEMFILE=Gemfile<variant> bundle exec rspec spec.
  • When you make a pull request, tests are automatically run against all variants and Rubies on

If you would like to contribute:

  • Fork the repository.
  • Push your changes with passing specs.
  • Send us a pull request.

I'm very eager to keep this gem lightweight and on topic. If you're unsure whether a change would make it into the gem, talk to me beforehand.


Tobias Kraze from makandra

Henning Koch from makandra