user_session.rb

class UserSession < Authlogic::Session::Base
  
  validate :check_presence
  
  # Without the following method, calling persisted? on UserSession fails,
  # which is needed for polymorphic_url when called with a UserSession.
  def destroyed?
    false
  end

  private
  
  # If neither email nor password are present, Authlogic adds an error to
  # :base, but not either field.
  def check_presence
    if !email && !password
      errors.add(:email, 'cannot be blank')
      errors.add(:password, 'cannot be blank')
    end
  end
  
end