Form objects example from https://forum.upcase.com/t/form-objects/2267

registration.rb

class Registration
  # Make this model play nice with `form_for`. Gives us validations, initialize, etc
  include ActiveModel::Model

  # Accessors for the fields we are exposing in the form
  attr_accessor :email, :password, :zip

  # This is an implementation detail of our authentication (Clearance). It's ultimately going to
  # sign this object in and that process expects a user object that has these fields available
  delegate :remember_token, :id, to: user

  # For this object to be valid, we want the child objects (user and profile) to be valid. This is
  # the validation method that does this.
  validate :validate_children

  # We tell registration it's a user. This will keep it pointing to Users controller and allow it to use
  # user translations
  def self.model_name
    User.model_name
  end

  # This is what our controller calls to save the user.
  def save
    if valid?
      # Create a transaction. If any of the database stuff in here fails, they will raise an exception
      # (because they are bang methods) and rollback the transaction.
      ActiveRecord::Base.transaction do
        user.save!
        profile.save!
      end
    end
  end

  private

  # Initialize the user object with the arguments that apply to user
  def user
    @user ||= User.new(email: email, password: password)
  end

  # initialize the profile object with the arguments that apply to profile
  def profile
    @profile ||= user.build_profile(zip: zip)
  end

  # the implementation of our validation method. We just delegate to our
  # two child objects which define their validations (presence on all fields in this case)
  def validate_children
    if user.invalid?
      promote_errors(user.errors)
    end

    if profile.invalid?
      promote_errors(profile.errors)
    end
  end

  # Errors on `user` or `profile` aren't helpful to us because `user.email` isn't on our form. `email` is however.
  # Take all errors from our child objects and promote them to the same field on the base object. This will make
  # them render properly on the form.
  def promote_errors(child_errors)
    child_errors.each do |attribute, message|
      errors.add(attribute, message)
    end
  end
end