Undefined Method with CanCan, Devise and STI roles

问题

I have setup Devise with STI roles and also CanCan (and rails admin). I am trying to make CanCan work why my STI roles here is my ability.rb files

def initialize(user)
  can :read, :all
  if user and user.is_admin?
    can :access, :rails_admin
    can :manage, :all
  end

I added this to my user.rb model

def is_admin?
  true if self.type == Admin
end

That code is inside the class User < ActiveRecord::Base

When I test I get this error

undefined method `is_admin?' for true:TrueClass

What am I missing here?

Answer in comments

来源：https://stackoverflow.com/questions/9527298/undefined-method-with-cancan-devise-and-sti-roles