rails4.0 中 user.has_role?('Admin') 这个方法，是 rails 自带的吗？
2.0.0-p247 :015 >user = User.find(1)
User Load (1.0ms) SELECT users.* FROM users WHERE users.id = ? LIMIT 1 [["id", 1]]
=> #
2.0.0-p247 :016 > user.has_role?('Admin')
Role Load (1.2ms) SELECT roles.* FROM roles INNER JOIN roles_users ON roles.id = roles_users.role_id WHERE roles_users.user_id = ? AND roles.name = 'Admin' LIMIT 1 [["user_id", 1]]
=> false
2.0.0-p247 :017 > role = Role.first
Role Load (0.4ms) SELECT roles.* FROM roles ORDER BY roles.id ASC LIMIT 1
=> #
2.0.0-p247 :018 > role.has_user?('Admin')
NoMethodError: undefined method `has_user?' for #...
