oracle的sequence自增序列在Eloquent里怎么指定？

model:
<?php
class Person extends Eloquent{

protected $table = 'person';
public $timestamps = false;
protected $guarded = array('*');//阻止所有字段被批量填充
protected $primaryKey = 'id';
}
?>
routes.php
Route::get('getFormOracle',function(){
$person = new Person();
$person->name = 'mickylo';
$person->nick_name ='micky';
$person->gender='1';
$person->age = 20;
$person->education='0';
$person->save();
});
按此插入数据会报
Error Code : 1400 Error Message : ORA-01400: 无法将 NULL 插入 ("THINK"."PERSON"."ID")
该怎么处理seqeunce的主键呢？
譬如，ID的序列为person_seq,如何指定?