Service Objects with ActiveModel and SimpleDelegator

Alireza Bashiri
4 min readApr 9, 2020

One of the worst practices in Ruby On Rails is to clutter the Gemfile to do basics as authentication, authorization, validation, etc. considering the fact that they are practiced for so long and they can be accomplished using just the batteries already included in Ruby On Rails; service objects are no different for which developers often end up using different gems to do such a simple pattern.

In the favour of simplicity and the fact that I never trust dependencies due to to the complexity they bring in I’ve been using ActiveModel and SimpleDelegator to apply service objects through a consistent interface across the controllers as well as avoiding dependency clutter and here’s the results.

Login case

Let’s consider a user logging in use case;

> Users can login with password and email
> Users cannot login with wrong password / email

So we have 2 cases which we have to cover in our implementation to do that I use ActiveModel::Validations to validate the parameters I receive from the controller and using save method to keep interface consistency.

class Login
include ActiveModel::Model
attr_accessor :email, :password, :user
validates :email, presence: true
validates :password, presence: true
validates :user, presence: {message: "email or password is wrong"}
def initialize(params)
super(params.require(:user).permit(:email, :password))
@user = User.find_by(email: email, status: :active)&.authenticate(password)
def save
return if invalid?
# This will generate and refresh user's token and return it

Here if no user found it will set @user variable to nil and user presence validation will handle the error which keep us from writing an error handling part except a message.

And the controller would be like;

class LoginController < ApplicationController
skip_before_action :authenticate
INCLUDED = %i[positions]
def create
login =
if (user =
render json: user, include: INCLUDED
render json: login, status: :bad_request
private def serializer
def user_params
params.require(:user).permit(:email, :password)

This is already battle-tested in production and it works like a charm.

Approval flow

Let’s consider a use case that your flow needs to go through multiple approvals and each approval stage has its own validations and business logic. There are a lot of ways to handle this;

ActiveRecord callbacks

It may seem simple solution but you have to write tons of conditions to avoid conflict between different use cases and hard to debug.

Contextual validations with on: :context

This works just fine but it’ll clutter your models with a lot of business logic which it needs to somewhere else and it’s hard to test and modify due to it’s coupled with your model. e.g.

validate :order_is_picked_before_packing, on: :packing

Decorators + State machines

State machines needs to be force-feed to developers as they make handling data logic so convenient but to provide a good interface for your controllers you cannot treat state machine transitions or guard errors as you best bet you still need something in-between which in this case is a decorator which give you best of both worlds validations, callbacks, separation of concern, ease of testing and etc. In our case we need some conditions are met before going to another state so the uses cases are;

> MVPs cannot be approved before they're submitted
> MVPs cannot be approved more than once by same user
> MVPs cannot be approved unless they have enough approvals from different users

Let’s take look at the code;

class ApprovedMvp < SimpleDelegator
include ActiveModel::Validations
def save(user)
return unless valid?
if in_state?(:draft)
errors.add(:minimal_viable_product, "it should be submitted first")
if approvals.find_by(user: user)
errors.add(:minimal_viable_product, "already been approved by user")
approvals.create!(user: user)
if has_enough_approvals?

Here we populate a decorated MVP with contextual errors and pass it up to the controller

class UseCases::MinimalViableProductsController < ApplicationController
INCLUDED = UseCasesController::INCLUDED
include UseCaseScoped def approve
result =
render json: result.use_case, include: INCLUDED
render json: result

In which will be handled by a ErrorSerializer and a concern called ActsAsJSONAPI which I’ve written to handle different cases of serialization and error handling without dependencies and a lot of complexity which you can checkout here.


Codes that are written 50 years ago with zero-dependencies are working fine today and the ones are written today with dependencies may not work tomorrow :)

And don’t get me wrong I don’t mean to reinvent the wheel but don’t be afraid to build your own tool since each app has different needs and not all gems/libraries are going to fit yours. Bad cases that you should never implement by yourself except for practice is encryption or an premature abstraction (framework, etc) that’s takes your more than 1 day or 2.

I hope you’ve enjoyed it


Feel free to checkout my personal blog here



Alireza Bashiri

∞ Travel | Coding | Lifestyle ⦿ Bangkok | ✧ #freelancer ▷ Got a project? ⭣