This is an extraction from Jim Weirich's "Decoupling from Rails" talk, which explained how to apply the hexagonal design pattern to make every layer of your application easily unit testable (without touching the database etc). It only seeks to extract a single method, the EmployeesController#create method, to illustrate the design damage that's …

test_induced_design_damage.rb

# Original Rails controller and action
class EmployeesController < ApplicationController
  def create
    @employee = Employee.new(employee_params)
    
    if @employee.save
      redirect_to @employee, notice: "Employee #{@employee.name} created"
    else
      render :new
    end
  end
end


# Hexagon-inspired, test-induced, damaged version
class EmployeesController < ApplicationController
  def create
    CreateRunner.new(self, EmployeesRepository.new).run(params[:employee])
  end
  
  def create_succeeded(employee, message)
    redirect_to employee, notice: message
  end
  
  def create_failed(employee)
    @employee = employee
    render :new
  end
end

class CreateRunner
  attr_reader :context, :repo
  
  def initialize(context, repo)
    @context = context
    @repo    = repo
  end
  
  def run(employee_attrs)
    @employee = repo.new_employee(employee_attrs)
    
    if repo.save_employee
      context.create_succeeded(employee, "Employee #{employee.name} created")
    else
      context.create_failed(employee)
    end
  end
end

class EmployeesRepository
  def new_employee(*args)
    Biz::Employee.new(Employee.new(*args))
  end
  
  def save_employee(employee)
    employee.save
  end
end

require 'delegate'

module Biz
  class Employee < SimpleDelegator
    def self.wrap(employees)
      employees.wrap { |e| new(e) }
    end
    
    def class
      __getobj__.class
    end
    
    # Biz logic ... AR is only a data-access object
  end
end

Let's all be civil.