Mongodb：如何按对象列表分组

我有一个下面的学生集合，它有列表可选，并且正在按列表聚合集合组

   {
    "_id" : "57e3e55c638cb8b971",
    "firstName" : "St1_firstName",
    "lastName" : "St1_lastName",
    "email" : "St1_firstname.lastName@xxx.com",
    "subjects" : [
        {
            "_id" : "35a344d888dd8b363",
            "code" : "00 00 00",
            "name" : "subject1"
        },
        {
            "_id" : "23a344d888dd8b335",
            "code" : "01 00 00",
            "name" : "subject2"
        },
        {
            "_id" : "45a344d888dd8b987",
            "code" : "01 01 00",
            "name" : "subject3"
        }
    ],
    "category" : {
        "_id" : "76a344d888dd8b887",
        "label" : "label"
    },
    "classroom" : {
        "_id" : "5877df01937e1f97942c9e06"
    }
}

下面是用于按列表聚合集合组的GORM代码段

DBObject unwind =  new BasicDBObject('_id',new BasicDBObject('$unwind','$subjects'))
        unwind.append('subjects','$subjects.code')
        unwind.append('count', new BasicDBObject('$sum', 1))
        DBObject group = new BasicDBObject('$group', unwind)
        queryPipe << group

这不起作用，你能更正代码吗 当我尝试使用以下完整代码时获取NullPointerException

List queryPipe = []
        DBObject matchclassroom = new BasicDBObject('$match', new BasicDBObject('classroom', params.classroom))
        queryPipe << matchclassroom

        final DBObject unwind = new BasicDBObject('$unwind', '$students')
        final DBObject groupFields = new BasicDBObject('_id', '$students.code')
        groupFields.put("count", new BasicDBObject('$sum', 1))
        final DBObject group = new BasicDBObject('$group', groupFields)
        queryPipe << group

        List facetsList = []
        Student.collection.aggregate(queryPipe).each { fct ->

                def studentsList = fct.'_id'.student

        }

---

您在代码结构方面犯了一些错误

DBObject unwind =  new BasicDBObject('_id',new BasicDBObject('$unwind','$subjects'))

这里，创建$unwind对象

DBObject group = new BasicDBObject('$group', unwind)

---

这里，创建$group对象。但是，您为$group对象的内部添加了$unwind对象。

我需要统计出现的次数，这是用于展开的方式。追加'count'，新的BasicDBObject'$sum'，1设置无效运算符'$unwind error'，即使我正在使用上述两行代码，为什么您要将unwind pipeline对象分配给$group pipeline对象，你能告诉我你遇到了什么错误吗？试着这样做，DBObject unwind=new basicdbobobject''u id'，new basicdbobobject'$unwind'，'$subjects'；DBObject group=new BasicDBObject['$group'：['\u id'：'$subjects.code'，'count'：['$sum'：1]]；AggregationOutput AggregationOutput=collection.aggregate[unwind，group]；aggregationOutput.results.each{dbObject->//对结果做点什么}在上述代码实现List queryPipe=[]dbObject matchroom=new BasicDBObject'$match'，new BasicDBObject'School'，params.planroom queryPipe然后Studen.collection.aggregatequeryPipe.each之前{fct->String subjectId=fct.''u id'。如果subjectId{subjectInfo=Subject.getsubjectId？.properties.subMap'id'，label'}facetsList，主题映射subjectInfo是否有任何更新？