Incomplete, but an idea about that DHH wrote https://gist.github.com/dhh/4849a20d2ba89b34b201

test_induced_design.rb

#RSPEC
require 'spec_helper'

describe EmployeesController do
  let(:employee_params) { ... }
  describe "POST create" do
    it "should create bla bla bla" do
      employee = mock_model(Employee, id: 666)
      Employee.should_receive(:create).with(employee_params).and_return employee
      post :create, employee: employee_params 
    end
  end
end

#RAILS CONTROLLER 
class EmployeesController < ApplicationController
  
  def create
    @employee = Employee.create(employee_params)
    respond_with @employee
  end
  
end