如何删除mongodb中组返回的文件？

Question

我是mongodb初学者,我正在做一个家庭作业问题,数据集看起来像这样

{ "_id" : { "$oid" : "50906d7fa3c412bb040eb577" }, "student_id" : 0, "type" : "exam", "score" : 54.6535436362647 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb578" }, "student_id" : 0, "type" : "quiz", "score" : 31.95004496742112 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb579" }, "student_id" : 0, "type" : "homework", "score" : 14.8504576811645 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb57a" }, "student_id" : 0, "type" : "homework", "score" : 63.98402553675503 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb57b" }, "student_id" : 1, "type" : "exam", "score" : 74.20010837299897 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb57c" }, "student_id" : 1, "type" : "quiz", "score" : 96.76851542258362 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb57d" }, "student_id" : 1, "type" : "homework", "score" : 21.33260810416115 }
{ "_id" : { "$oid" : "50906d7fa3c412bb040eb57e" }, "student_id" : 1, "type" : "homework", "score" : 44.31667452616328 }

作为问题的一部分,我必须为每个学生删除分数最低的"家庭作业"文档.这是我的策略

在聚合管道
1中:首先过滤所有类型的文档:homeworks
2:按student_id排序,得分
3:在student_id上做一个组,找到第一个元素

这将给我所有得分最低的文件,

但是我如何从原始数据集中删除这些元素？任何指导或提示？

Answer 1

使用聚合中的游标结果使用游标forEach()方法循环遍历文档,然后使用方法中_id的查询从集合中删除每个文档remove().像这样的东西:

var cursor = db.grades.aggregate(pipeline);
cursor.forEach(function (doc){
    db.grades.remove({"_id": doc._id});
});

另一种方法是_id使用该map()方法创建文档的数组,并删除以下文档:

var cursor = db.grades.aggregate(pipeline),
    ids = cursor.map(function (doc) { return doc._id; });
db.grades.remove({"_id": { "$in": ids }});

- 更新 -

对于大型删除操作,将要保留的文档复制到新集合然后drop()在原始集合上使用可能更有效.要复制基本文档,您的聚合管道需要返回没有最低作业文档的文档,并使用$out运算符作为最终管道阶段将它们复制到另一个集合.考虑以下聚合管道:

db.grades.aggregate([    
    {
        '$group':{
            '_id': {
                "student_id": "$student_id",
                "type": "$type"
            },
            'lowest_score': { "$min": '$score'},
            'data': {
                '$push': '$$ROOT'
            }
         }
    },    
    {
        "$unwind": "$data"
    },
    {
        "$project": {
            "_id": "$data._id",
            "student_id" : "$data.student_id",
            "type" : "$data.type",
            "score" : "$data.score",
            'lowest_score': 1,            
            "isHomeworkLowest": {
                "$cond": [
                    { 
                        "$and": [
                            { "$eq": [ "$_id.type", "homework" ] },
                            { "$eq": [ "$data.score", "$lowest_score" ] }
                        ] 
                    },
                    true,
                    false
                ]
            }
        }
    },
    {
        "$match": {"isHomeworkLowest" : false}
    },
    {
        "$project": {           
            "student_id": 1,
            "type": 1,
            "score": 1
        }
    },
    {
        "$out": "new_grades"
    }
])

然后,您可以在其中删除旧集合db.grades.drop(),然后查询db.new_grades.find()

Answer 2

我认为这是MongoDB大学提供的MongoDB for Java Developers的家庭作业的数据库部分.要求是删除每个学生的最低分.无论如何,我这样解决了.我希望它对你有所帮助.你也可以从我的github链接克隆我的代码(下面提供)

public class Homework2Week2 {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    // Here the the documentation is used for mongo-jva-driver-3.2.2.jar
    /*If you want to use different versionof  mongo-jva-driver 
      then you have look for that version specificatios.*/
    MongoClient mongoClient = new MongoClient();
    // get handle to "students" database
    MongoDatabase database = mongoClient.getDatabase("students");
    // get a handle to the "grades" collection
    MongoCollection<Document> collection = database.getCollection("grades");
    /*
     * Write a program in the language of your choice that will remove the grade of type "homework" with the lowest score for each student from the dataset in the handout. 
     * Since each document is one grade, it should remove one document per student. 
     * This will use the same data set as the last problem, but if you don't have it, you can download and re-import.
     * The dataset contains 4 scores each for 200 students.
     * First, letâs confirm your data is intact; the number of documents should be 800.

     *Hint/spoiler: If you select homework grade-documents, sort by student
      and then by score, you can iterate through and find the lowest score
      for each student by noticing a change in student id. As you notice
      that change of student_id, remove the document.
     */
    MongoCursor<Document> cursor = collection.find(eq("type", "homework")).sort(new Document("student_id", 1).append("score", 1)).iterator();
    int curStudentId = -1;
    try
    {
    while (cursor.hasNext()) {
        Document doc = cursor.next();
        int studentId=(int) doc.get("student_id");
        if (studentId != curStudentId) {
            collection.deleteMany(doc);
            curStudentId = studentId;
        }
    }
    }finally {
        //Close cursor
        cursor.close();
    }   
    //Close mongoClient
    mongoClient.close();
}

}

在我的Github帐户中,我有完整的项目代码.如果有人想要你可以试试这个链接.

Answer 3

db.grades.aggregate( [ 
                            { 
                                $match:{type:'homework'}
                            }, 
                            { $group: 
                                 { _id: {student_id:"$student_id",type:'$type'},                                   
                                   score: { $max: "$score" } 
                                 } 
                            } 
                            ]).forEach(function(doc){
db.grades.remove({'student_id':doc._id.student_id,'score':doc.score})

})