group-by - MongoDB对对聚合查询结果进行筛选

Question

想要用java做分组查询，貌似2.4版的java驱动包不支持db.collection.aggregate语法，所以被迫改用db.collection.group()方法。
需求：要用Mongo实现该与SQL的相同作用：
select number from user group by number having count(*) = 1

使用aggregate可以这么实现：

db.user.aggregate(
 {$group:{_id:"$number",total:{$sum:1}}},
 {$match:{total:1}}
);

现在改用group方法：

db.user.group({
  key : {number:1}, 
  cond : {count:1}, 
  initial : {count: 0}, 
  reduce : function Reduce(doc, out) {
   out.count +=1;
  }, 
  finalize : function Finalize(out) {
   return out;
  }
});

查不出结果，如何解决？

Answer 1

I tried another method, but it still didn’t work

db.user.group({
  key : {number:1}, 
  cond : {}, 
  initial : {count: 0}, 
  reduce : function Reduce(doc, out) {
   out.count +=1;
  }, 
  finalize : function Finalize(out) {
   if(out.count==1){
        return out;
    }
  }
});

By the way, is there any use for return in finalize?

Answer 2

2.4 was released in 2010. The MongoDB server released at that time is no longer officially supported, so it is necessary to upgrade.

Answer 3

For aggregation, version 3.2 or above is recommended. It is recommended to use https://github.com/T-baby/Mon... to operate and manage mongodb. Please refer to https://t-baby.gitbooks.io/mo... . Get started in ten minutes