Example of using Casting Gem.

purchase.rb

class Purchase
  include ActiveModel::Model
  
  attr_accessor :customer # + other order related fields..
  
  # some validations
  
  def save
    if valid?
      charge_customer && create_order
    else
      false
    end
  end
  
  private
  
  attr_reader :charge
  
  def charge_customer
    @charge = customer.create_charge(...) # the `create_charge` method is from `Customer` module
  rescue ChargeFailedError => e
    errors.add(...)
    false
  end
  
  def create_order
    customer.orders.create!(...)
  end
end

purchase_spec.rb

require 'spec_helper'

describe Purchase, '#save' do
  let(:customer) { double('Customer', create_charge: true) }
  
  it 'charges the customer' do
    Purchase.new(purchase_params).save
    
    expect(customer).to have_received(:create_charge).with(...)
  end
  
  it 'perists the order' do
    # ...
  end
  
  def purchase_params(options = {})
    {
      # ...
    }.merge(options)
  end
end

purchases_controller.rb

class PurchasesController < ApplicationController
  def create
    # service object: charges customer, persists the Order to DB
    purchase = Purchase.new(purchase_params)   
    purchase.save
    
    respond_with purchase
  end
  
  private
  
  def purchase_params
    params.require(:purchase).permit(
      # product, price, etc  
    ).merge(
      customer: customer  
    )
  end
  
  def customer
    current_user.cast_as(Customer)
  end
end