user_steps.rb

module UserHelpers
  def current_user_session
    return @current_user_session if defined?(@current_user_session)
    @current_user_session = UserSession.find
  end

  def current_user
    return @current_user if defined?(@current_user)
    @current_user = current_user_session && current_user_session.user
  end
end
World(UserHelpers)

def user
  @user ||= Factory(:user)
end

def login
  user
  visit '/user_session/new'
  fill_in("Login", :with => user.login)
  fill_in("Password", :with => user.password)
  click_button("Login")
end

Given /^I am a logged in user$/ do
  login
end