使用Java从mongoDB中的数组中删除子文档

Question

Im taking the MongoDB University tutorial class right now. In one of my assignments I have to find the lowest homework score in an array of subdocuments (apologies if im using incorrect syntax) and remove that subdocument. So far my code has only worked to remove the lowest homework score of the first document, it just iterates over the other 199 after that but doesnt remove anything. I'm still pretty new to this stuff so I'm not sure where I am going wrong.

Here is an example of what a document first looks like:

{
"_id" : 100,
"name" : "Demarcus Audette",
"scores" : [
    {
        "score" : 47.42608580155614,
        "type" : "exam"
    },
    {
        "score" : 44.83416623719906,
        "type" : "quiz"
    },
    {
        "score" : 39.01726616178844,
        "type" : "homework"
    },
    {
        "score" : 56.73927384092820,
        "type" : "homework"
    }
  ]
 }

Here is my code

public class MongoTest
{

public static void main(String[] args) throws UnknownHostException
{
    MongoClient client = new MongoClient();
    DB db = client.getDB("school");
    DBCollection c = db.getCollection("students");

    QueryBuilder builder = new QueryBuilder().start("scores.type").is("homework");
    DBCursor cursor = c.find(builder.get());

    List<DBObject> list;


    list = cursor.toArray();
    int i = 1;

    for (DBObject dbObject : list)
    {

        System.out.println(dbObject.toString());

        String info = dbObject.toString();
        String[] infoList = info.split(",");

        double[] scoreList = new double[2];
        String[] infoList2 = infoList[7].split(":");
        String[] infoList3 = infoList[infoList.length-1].split(":");

        char[] word = infoList2[1].toCharArray();
        char[] word2 = Arrays.copyOfRange(word, 0, word.length-3);

        String temp = String.valueOf(word2);

        char[] word3 = infoList3[1].toCharArray();
        char[] word4 = Arrays.copyOfRange(word3, 0, word3.length-3);

        String temp2 = String.valueOf(word4);


        double num = Double.parseDouble(temp);
        double num2 = Double.parseDouble(temp2);



                System.out.println(num);
                System.out.println(num2);

                BasicDBObject match = new BasicDBObject("_id", i); //to match your direct app document
                BasicDBObject update = new BasicDBObject("score", num);
                BasicDBObject update2 = new BasicDBObject("score", num2);


                if(num>num2)
                {
                    System.out.println("The first score is higher");
                    c.update(match, new BasicDBObject("$pull", update2));
                } 
                if(num2>num)
                {
                    System.out.println("The second score is higher!");
                    c.update(match, new BasicDBObject("$pull", update));
                }

        i++;
    }


    System.out.println("\ncount: " + c.count());

}

I can get the code to correctly compare scores but then I cant get it to remove anything. Thanks for any help as to what im missing.

Answer 1

Small correction needed in your code. You need to provide entire sub document in the as such in the array:

BasicDBObject update2 = new BasicDBObject("type",
                                          "homework").append("score", num2);
 if (num < num1) {
  System.out.println("The second score is higher");
  collection.update(match, new BasicDBObject("$pull", 
                          new BasicDBObject("scores", update1)));
}

This should work:)