问使用节点js的Mongodb嵌套查询

EN

var acquireFriendsPositions = function(db, id, res, callback) {
  var cursor = db.collection('users').find({"_id" : new ObjectId(id)}, {_id:0, friends:1});
  cursor.each(function(err, doc) {
      assert.equal(err, null);
      if (doc != null) {
         friendsP = doc.friends;
         console.log(friendsP);   //I get the array friendsP
         for(var i =0; i<friendsP.length; i++)
         {
          console.log(friendsP[i]);   //friendsP[i] has proper value
           var curs = db.collection('users').find({"_id" : new ObjectId(friendsP[i])});   //but query returns null
           curs.each(function(err, loc) {
            //assert.equal(err, null);
            if(loc!= null) {
                console.log(loc);
                friendsPos.push(loc);
              }
              else {
                console.log("else");
              }
           });
         }
        promise(friendsPos, res); //here i wait for friendsPos and use it in res.send(), but friendsPos is empty because loc is always null
      } else {
         callback();  //callback does db.close();
      }
   });
};