user_form.rb

class User < ApplicationRecord
  has_one :car
  
  validates :username, presence: true,
  validates :username, length: { maximum: 100 }
  validates :age, numericality: { only_integer: true }
end

class Car < ApplicationRecord  
  enum vehicle_type: %i[suv van compact]
  
  belongs_to :user
  
  validates :user, presence: true
  validates :model, presence: true
end

class UserAdultForm < BaseForm
  USER_ATTRIBUTES = %i[username age]
  CAR_ATTRIBUTES = %i[model vehicle_type]
  
  attr_accessor(*USER_ATTRIBUTES)
  attr_accessor(*CAR_ATTRIBUTES)
  
  validates :age, numericality: { greater_than_or_equal_to: 18 }
  
  def initialize(params = {})
    @user = initialize_user(params)
    @car = initialize_car(params)
    
    super(
      params.merge(
        user.slice(*USER_ATTRIBUTES)
      ).merge(
        car.slice(*CAR_ATTRIBUTES)
      )
    )
  end
  
  def valid?
    errors.clear
    
    super
    
    user.valid?
    delegate_validation_on(user, :username)
    delegate_validation_on(user, :age)
    
    car.valid?
    delegate_validation_on(car, :model)
    delegate_validation_on(car, :vehicle_type)
    
    errors.empty?
  end
  
  def vehicle_types
    Car.vehicle_types.keys
  end
  
  private
  
  attr_reader :user
  attr_reader :car
  
  def persist!
    ActiveRecord::Base.transaction do
      user.save!
      car.save!
    end
  end
  
  def initialize_user(params)
    user = params[:id] ? User.find(params[:id]) : User.new
    user.attributes = params.slice(*USER_ATTRIBUTES)
    user
  end
  
  def initialize_car(params)
    car = user.car || user.build_car
    car.attributes = params.slice(*CAR_ATTRIBUTES)
    car
  end
end