Idiorm和巴黎协会定制外键

i've tested Idiorm and Paris library and i found a strange behaviour when i use association with custom foreign_key This is the code

Class User extends Model{

public static $_table = 'User';
public static $_id_column = 'UserId';

/*
 * Associations
 */
 public function department() {
    return $this->has_one('Department','DepartmentId');
}

} Class Department extends Model{

public static $_table = 'Department';
public static $_id_column = 'DepartmentID';

/*
 * Association
 */
public function user() {
    return $this->belongs_to('User','UserID');
}

} Custom foreign key is mandatory because the database schema doesn't modifiable. the result on query log test: $department= Model::factory('Department')->find_one(23); $user = $department->user()->find_many(); is: SELECT * FROM User WHERE UserId = '' LIMIT 1 why?

strong text sorry, my mistake in order of relation i configure in a wrong way has_one and belongs_to the correct code invert the relation