Topic: alias_attribute with association

I am using alias_attribute to map some models to an existing database and confused why the following does not work:

class User < ActiveRecord::Base
  set_table_name :UserData
  set_primary_key :UserDataID

  alias_attribute :id, :UserDataID
  alias_attribute :person_id, :PersonID

  belongs_to :person
end
class Person < ActiveRecord::Base
  set_table_name   :Persons
  set_primary_key  :PersonID

  alias_attribute  :id, :PersonID

  has_many :users
end

When I use the console to access all of a persons users I see that the alias for person_id is not being used.

irb(main):001:0> Person.first.users
  Person Load (0.1ms)  SELECT `Persons`.* FROM `Persons` LIMIT 1
  User Load (22.9ms)  SELECT `UserData`.* FROM `UserData` WHERE `UserData`.`person_id` = 1
ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'UserData.person_id' in 'where clause': SELECT `UserData`.* FROM `UserData`  WHERE `UserData`.`person_id` = 1

I can add in the foreign key explicitly in my associations and it works but I am wondering why the alias alone is not enough?