Mongodb Java：根据元素的某些条件删除文档中嵌入数组的元素

Question

我有一个学生文档列表，其结构如下：

{
    "_id" : 0,
    "name" : "aimee Zank",
    "scores" : [
            {
                    "type" : "exam",
                    "score" : 1.463179736705023
            },
            {
                    "type" : "quiz",
                    "score" : 11.78273309957772
            },
            {
                    "type" : "homework",
                    "score" : 6.676176060654615
            },
            {
                    "type" : "homework",
                    "score" : 35.8740349954354
            }
    ]
}

正如您所看到的，每个学生都有一个包含 4 个分数的列表。我需要删除每个学生文档的最低“家庭作业”分数。每个学生都有 2 个“homewok”类型分数条目（4 个元素数组中的最后 2 个条目）。分数类型的架构和顺序是一致的，并且对于所有学生都有相同的模式，感谢您的帮助。

这是我迄今为止试图实现的目标：

    DBCursor cursor = collection.find();

    try {

        while(cursor.hasNext()) {
           BasicDBObject doc = (BasicDBObject) cursor.next();
           BasicDBList scoreList =  (BasicDBList) doc.get("scores");

           BasicDBObject hw1 = (BasicDBObject) scoreList.get("2");
           double hw1Score = hw1.getDouble("score");

           BasicDBObject hw2 = (BasicDBObject) scoreList.get("3");
           double hw2Score = hw2.getDouble("score");

           if (hw1Score > hw2Score) {
               BasicDBObject update = new BasicDBObject("scores.score", hw2Score);               
               collection.update(doc, new BasicDBObject("$pull",update));                                                  
           } else {
               BasicDBObject update = new BasicDBObject("scores.score", hw1Score);

               collection.update(doc, new BasicDBObject("$pull",update));                                  
           }
           System.out.println(doc);
        }

    } finally {
       cursor.close();
    }


}

Answer 1

这里的所有答案都很棒。我只是想补充一点，如果有人想使用 Java 操作符（自驱动程序 v3.1 起），那么他可以执行以下操作，而不是使用“$pull”操作符：

...
Bson studentFilter = Filters.eq( "_id", doc.get("_id") );
Bson delete = Updates.pull("scores", new Document("score", lowestHomework).append("type", "homework"));
collection.updateOne(studentFilter, delete);

我认为它更优雅。所以我的完整答案是：

public static void main(String[] args) {
    MongoClient client = new MongoClient();
    MongoDatabase database = client.getDatabase("school");
    MongoCollection<Document> collection = database.getCollection("students");

    List<Document> homeworks = collection.find()
        .into(new ArrayList<Document>());

    for(Document doc : homeworks)
    {
        ArrayList<Document> scores = (ArrayList<Document>) doc.get("scores");
        //iterate over the scores of each student (there are 4 scores: quiz, exam and 2*homework)
        double lowestHomework = Double.MAX_VALUE;
        for(Document embeddedDoc : scores)
        {
            if(embeddedDoc.getString("type").equals("homework"))
            {
                Double score = embeddedDoc.getDouble("score");
                if(score < lowestHomework)
                {
                    lowestHomework = score;
                }
            }
        }
        Bson studentFilter = Filters.eq( "_id", doc.get("_id") );
        Bson delete = Updates.pull("scores", new Document("score", lowestHomework).append("type", "homework"));

        collection.updateOne(studentFilter, delete);
    }

    client.close();
}