node.js - 我如何在 Mongoose 聚合中使用字段值？ ( Node )

Question

如何在 Mongoose 聚合中使用字段值？

我尝试使用最新版本的 mongodb、mongoose。

Group.find({name:'1'},function(err,groups){
    groups.forEach(function(g){
        result=await People.countDocuments({group:g._id})
    });
});

我已经检查过这段代码的“结果”是 3, 5, 2

我想使用聚合

Group.aggregate()
        .project({_id:true,name:true})
        .addFields({numOfPeople: await People.countDocuments({group:this._id})});

输出

[{name:'1',numOfPeople:0},
{name:'2',numOfPeople:0},
{name:'3',numOfPeople:0}]

预计

[{name:'1',numOfPeople:3},
{name:'2',numOfPeople:5},
{name:'3',numOfPeople:2}]

Answer 1

您必须使用分组依据

db.groups.aggregate(
   [
      {
        $group : {
           _id : "$name",
           numOfPeople: { $sum: 1 }
        }
      }
   ]
);