How to stub ApplicationController method in request spec

skalee seems to have provided the correct answer in the comment.

If the method you're trying to stub is an instance method (most likely) and not a class method then you need use:

ApplicationController.any_instance.stub(:current_user)

Here are a couple of examples of the basic form.

controller.stub(:action_name).and_raise([some error])
controller.stub(:action_name).and_return([some value])

In your particular case, I believe the proper form would be:

controller.stub(:current_user).and_return([your user object/id])

Here's a full working example from a project I work on:

describe PortalsController do

  it "if an ActionController::InvalidAuthenticityToken is raised the user should be redirected to login" do
    controller.stub(:index).and_raise(ActionController::InvalidAuthenticityToken)
    get :index
    flash[:notice].should eql("Your session has expired.")
    response.should redirect_to(portals_path)
  end

end

To explain my full example, basically what this does is verify that, when an ActionController::InvalidAuthenticityToken error is raised anywhere in the app, that a flash message appears, and the user is redirected to the portals_controller#index action. You can use these forms to stub out and return specific values, test an instance of a given error being raised, etc. There are several .stub(:action_name).and_[do_something_interesting]() methods available to you.

Update (after you added your code): per my comment, change your code so it reads:

require 'spec_helper'

describe "Login" do

   before(:each) do
      @mock_controller = mock("ApplicationController") 
      @mock_controller.stub(:current_user).and_return(User.first)
   end

  it "logs in" do
    visit '/'
    page.should have_content("Hey there user!")
  end

end

This works for me and gives me a @current_user variable to use in tests.

I have a helper that looks like this:

def bypass_authentication
  current_user = FactoryGirl.create(:user)

  ApplicationController.send(:alias_method, :old_current_user, :current_user)
  ApplicationController.send(:define_method, :current_user) do 
    current_user
  end
  @current_user = current_user
end

def restore_authentication
  ApplicationController.send(:alias_method, :current_user, :old_current_user)
end

And then in my request specs, I call:

before(:each){bypass_authentication}
after(:each){restore_authentication}