Caching 'false' in an instance variable in ruby (rails)

instance_var_caching.rb

# this only gets cached in @admin if true
# how to cache this also in case of false?

def admin?
  @admin ||= has_role? 'admin'
end


#is the following ok?

def admin?
  return @admin if defined? @admin
  @admin ||= has_role? 'admin'
end