Controller specs. http://blog.codeship.io/2013/12/16/yes-you-should-write-controller-tests.html

users_controller_spec.rb

require 'spec_helper'

describe UsersController, '#create' do
  let(:group_id) { double(:group_id) }
  let(:group) { double(:group) }
  let(:user) { double(:user) }

  before do
    Group.should_receive(:find).with(group_id).and_return(group)
    group.should_receive(:create_user).with(attributes).and_return(users)
  end

  context ‘when attributes are valid’ do
    it ‘saves the user and redirects to the index page’ do
      user.should_receive(:persisted?).and_return(true)
      
      post :create, group_id: group_id, user: attributes
      
      expect(subject).to redirect_to(:users)
    end
  end

  context ‘when attributes are not valid’ do
    it ‘renders the new template’ do
      user.should_receive(:persisted?).and_return(false)
      
      post :create, group_id: group_id, user: attributes
      
      expect(subject).to render_template(:new)
    end
  end
  
  context ‘when group is not found’ do
    it 'renders the not_found template' do
      Group.should_receive(:find).with(group_id).and_raise(ActiveRecord::RecordNotFound)
      
      expect(subject).to render_template(:not_found)
    end
  end
end