Update

Updating records involves modifying existing documents in the database. MongoRepository provides methods to perform these updates, such as save() (which updates if the document exists), and custom query methods.

save (Update a Single Document)

To update a single document, you can use the save() method. This method will update the document if it already exists based on its id, or insert it if it does not exist.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class StudentService {
    @Autowired
    private StudentRepository repository;

    public Grade updateGrade(Grade grade) {
        return repository.save(grade);
    }
}

See if you can write a method that adds a comment to an existing grade in the Update.java class.

upsert

Upsert refers to updating an existing document or inserting it if it does not exist. This can be achieved using the save() method as well.

To upsert a grade based on studentId and classId:

@Autowired
private StudentRepository repository;

public void upsertGrade(double studentId, double classId, Grade grade) {
    Grade existingGrade = repository.findByStudentIdAndClassId(studentId, classId);
    if (existingGrade == null) {
        grade.setId(new ObjectId());
        grade.setStudentId(studentId);
        grade.setClassId(classId);
        grade.setScores(List.of(new Score("homework", 50d)));
        grade.setComment("You will learn a lot if you read the MongoDB blog!");
    } else {
        existingGrade.getScores().add(new Score("quiz", 70d));
    }
    Grade upsertedGrade = repository.save(existingGrade != null ? existingGrade : grade);
    System.out.println("Upserted grade: " + upsertedGrade);
}

While the provided method for upserting works, it can be further optimized and made more idiomatic to MongoDB operations. Using the save method in Spring Data MongoDB works, but it involves fetching the document first, which can be suboptimal.

upsert with MongoTemplate

A more efficient way to perform upserts is by using MongoDB's upsert functionality directly in a repository query. This way, you avoid the need to fetch the document first, reducing the number of database operations.

Here's how you can do it using MongoTemplate for more control and efficiency:

@Autowired
private MongoTemplate mongoTemplate;

public void upsertGrade(double studentId, double classId, Grade grade) {
    Query query = new Query();
    query.addCriteria(Criteria.where("studentId").is(studentId)
            .and("classId").is(classId));

    Update update = new Update();
    update.set("studentId", studentId);
    update.set("classId", classId);
    update.set("comment", "You will learn a lot if you read the MongoDB blog!");
    update.addToSet("scores").each(new Score("homework", 50d), new Score("quiz", 70d));

    mongoTemplate.upsert(query, update, Grade.class);
}

saveAll (Update Multiple Documents)

To update multiple documents at once, use the saveAll() method provided by the StudentRepository. Pass a list of Grade objects to this method, and it will update all the grades in a single batch operation.

To update all grades with studentId greater than or equal to 10001:

@Autowired
private StudentRepository repository;

public void updateGradesForStudentIdGreaterThanEqual(double studentId) {
    List<Grade> grades = repository.findByStudentIdGreaterThanEqual(studentId);
    grades.forEach(g -> g.setComment("Updated for all grades with student_id >= 10001"));
    repository.saveAll(grades);
    System.out.println("Updated all grades with student_id >= 10001.");
}

Find and Update

To find a document and then update it, you first retrieve the document, modify it, and then save it back to the database.

To find a grade by studentId and update it:

@Autowired
private StudentRepository repository;

public void findAndUpdateGrade(double studentId, String newComment) {
    Grade grade = repository.findByStudentId(studentId);
    if (grade != null) {
        grade.setComment(newComment);
        Grade updatedGrade = repository.save(grade);
        System.out.println("Updated grade after finding: " + updatedGrade);
    }
}