I wanted a #login method in test_helper that would allow me to easily login from any of my functional tests. However, the #post method won’t allow you to set a different controller than the one in the @controller instance variable that’s defined in your test’s #setup. Well, by looking at how the #process method works, you can see that it just grabs the controller from @controller. Redefine that, and you’re good to go: 

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old_controller = @controller
@controller = LoginController.new
post(
  :attempt_login,
  {:user => {:name => 'joe', :password => 'password'}}
)
@controller = old_controller

If you have several login methods, such as a #login_admin and #login_regular, you could make a wrapper to reduce duplication:

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def wrap_with_controller( new_controller = LoginController )
  old_controller = @controller
  @controller = new_controller.new
  yield
  @controller = old_controller
end

def login_admin
  wrap_with_controller do
    post(
      :attempt_login,
      {:user => {:name => 'root', :password => 'password'}}
    )
  end
end

def login_regular
  wrap_with_controller do
    post(
      :attempt_login,
      {:user => {:name => 'joe', :password => 'password'}}
    )
  end
end