mongodb高级修改问题

Question

描述：格式如下所示，其中每个对象有_id，name，和一个数组scores，其中可以看到修改前的数组中，每个document有两个type为"homework"的对象。
*提问*：问题是如何操纵mongo数据库，批量修改db.students，让每个document中，删除score较小的homework，而保留score较大的homework。
修改前：
{
"_id":100,
"name":"DemarcusAudette",
"scores":[
{
"score":47.42608580155614,
"type":"exam"
},
{
"score":44.83416623719906,
"type":"quiz"
},
{
"score":19.01726616178844,
"type":"homework"
},
{
"score":39.01726616178844,
"type":"homework"
}
]
}
修改后：
{
"_id":100,
"name":"DemarcusAudette",
"scores":[
{
"score":47.42608580155614,
"type":"exam"
},
{
"score":44.83416623719906,
"type":"quiz"
},
{
"score":39.01726616178844,
"type":"homework"
}
]
}
下面附上一段nodejs上跑的代码（自己写的，有问题跑不通，作为参考）：
varMongoClient=require('mongodb').MongoClient;
MongoClient.connect('mongodb://localhost:27017/school',function(err,db){
if(err)throwerr;
varquery={};
varcursor=db.collection('students').find(query);
cursor.each(function(err,doc){
if(err)throwerr;
if(doc==null){returndb.close();}
/*TODO*/
vartarget1=doc.scores[2];
vartarget2=doc.scores[3];
if(target1elsedoc.update({$unset:target2});
console.dir("Successfullyfound"+target1);
});
});