mongodb中的$ graphLookup没有返回结果

Question

基本上我有这种类型的数据，我希望使用$ graphLookup

获得关系

{_id: "111", pin: "8018", introducer: ""}
{_id: "222", pin: "042f", introducer: "8018"}
{_id: "333", pin: "1234", introducer: "042f"}

我的输出应该是

{_id: "111", pin: "8018", introducer: "", treeView: []}
{_id: "222", pin: "042f", introducer: "8018", treeView: [{_id: "111", pin: "8018", introducer: ""}]}
{_id: "333", pin: "1234", introducer: "042f", treeView: [{_id: "111", pin: "8018", introducer: ""},{_id: "222", pin: "042f", introducer: "8018"} ]}

我的代码

userSchema.statics.getChildren = function(user){
return this.aggregate([
    { $graphLookup: {            
        from: "userSchema",
        startWith: "$introducer",
        connectFromField: "introducer",
        connectToField: "pin",
        as: "treeView"       
        }
    }
]);

};

但是我只获得了treeView的空白数组。请帮忙。