求一个聚合查询

求一个聚合查询的方法，自己折腾了好久都没折腾出来。。提前谢谢
有一个商品分类表，用于存放一级于二级分类，一级类目parent_id为空，数据如下：

{  
    "_id":"18ed09686190fe91052e14331649a71a",  
    "name":"一级类目1",  
    "parent_id":""  
},  
{  
    "_id":"fa24ce1a61910815066eec09203fa9b2",  
    "name":"二级类目1-1",  
    "parent_id":"18ed09686190fe91052e14331649a71a"  
},  
{  
    "_id":"fa24ce1a619108b2066f33a76a95da05",  
    "name":"二级类目1-2",  
    "parent_id":"18ed09686190fe91052e14331649a71a"  
},  
{  
    "_id":"18ed09686190fe91052e14331649a71b",  
    "name":"二级类目1",  
    "parent_id":""  
},  
{  
    "_id":"fa24ce1a61910815066eec09203fa9b3",  
    "name":"二级类目2-1",  
    "parent_id":"18ed09686190fe91052e14331649a71b"  
},  
{  
    "_id":"fa24ce1a619108b2066f33a76a95da06",  
    "name":"二级类目2-2",  
    "parent_id":"18ed09686190fe91052e14331649a71b"  
}

如何查询输出成如下格式：

[  
    {  
        "_id":"18ed09686190fe91052e14331649a71a",  
        "name":"一级类目1",  
        "parent_id":"",  
        "children":[  
            {  
                "_id":"fa24ce1a61910815066eec09203fa9b2",  
                "name":"二级类目1-1",  
                "parent_id":"18ed09686190fe91052e14331649a71a"  
            },  
           {  
                "_id":"fa24ce1a61910815066eec09203fa9b2",  
                "name":"二级类目1-2",  
                "parent_id":"18ed09686190fe91052e14331649a71a"  
            }  
        ]  
    },  
    {  
        "_id":"18ed09686190fe91052e14331649a71b",  
        "name":"一级类目2",  
        "parent_id":"",  
        "children":[  
            {  
                "_id":"fa24ce1a61910815066eec09203fa9b2",  
                "name":"二级类目2-1",  
                "parent_id":"18ed09686190fe91052e14331649a71a"  
            },  
           {  
                "_id":"fa24ce1a61910815066eec09203fa9b2",  
                "name":"二级类目2-2",  
                "parent_id":"18ed09686190fe91052e14331649a71a"  
            }  
        ]  
    }  
]