controller_spec.rb

describe MyController, '#index' do
  include PureControllerTestHelper

  let(:user) { MyModel.make_unsaved }

  before do
    mock(MyModel).find { user }
  end

  it 'should assign @user' 
    expect { controller.index }.to change { assigns[:user] }.from(nil).to(user)
  end
end

pure_controller_test_helper.rb

module PureControllerTestHelper
  def self.included(target)
    target.instance_eval do
      before do
        @_assigns_hash_proxy = Spec::Rails::Example::AssignsHashProxy.new(self) { controller }
        
        controller.send(:initialize_template_class, response)
        controller.send(:assign_shortcuts, request, response)
      end
    end
  end
end