In Alter you can specify relations between models such as belongs_to and has_many.

Creates the Car model:

use Hero\Core\Model;
class Car extends Model {

}

Attachs a field of type checkbox_list with a list of cars into the User model:

use Hero\Core\Model;
class User extends Model {

  public $has_many = 'car';

}

Attachs a field of type list with a list of users into the Apartment model:

use Hero\Core\Model;
class Apartment extends Model {

  public $belongs_to = 'user';

}