Mapping POJOs

In Spring Data MongoDB, POJOs (Plain Old Java Objects) are mapped to MongoDB documents using annotations. This mapping allows for the conversion between Java objects and MongoDB documents seamlessly. Here’s a step-by-step guide on how POJOs are mapped with Spring Data MongoDB:

Define the POJO Class

Start by defining your POJO class that represents the MongoDB document. Use annotations to specify how the fields of the class are mapped to the fields of the MongoDB document.

package com.mongodb.quickstart.models;

import org.bson.types.ObjectId;
import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.Document;
import org.springframework.data.mongodb.core.mapping.Field;

import java.util.List;

@Document(collection = "grades")
public class Grade {

    @Id
    private ObjectId id;

    @Field("studentId")
    private String studentId;

    private Double classId;
    private List<Score> scores;
    private String comment;

    // Getters and Setters
}

Use Annotations for Mapping

Annotations help Spring Data MongoDB understand how to map the fields in your Java class to the fields in the MongoDB document:

• @Document: Marks a class as being a MongoDB document. The collection attribute specifies which collection to store the document in.
• @Id: Marks a field as the primary key (unique identifier) of the document.
• @Field: Maps a Java field to a field in the MongoDB document.

Nested Objects

You can also map nested objects within a document. For example, if your Sale document contains a Customer object and a list of Item objects, define these nested classes separately and use them in your main POJO.

public class Score {
    private String type;
    private Double score;

    // Getters and Setters
}

Repository Definition

We now circle back to our repository. Define a repository interface to perform CRUD operations on your MongoDB documents. Spring Data MongoDB provides the MongoRepository interface for this purpose.

package com.mongodb.quickstart.repositories;

import org.springframework.data.mongodb.repository.MongoRepository;
import com.mongodb.quickstart.models.Sale;
import org.springframework.stereotype.Repository;

@Repository
public interface StudentRepository extends MongoRepository<Grade, String> {
    // Additional query methods can be defined here
}

By using these annotations and repository interfaces, Spring Data MongoDB makes it easy to map POJOs to MongoDB documents and perform database operations with minimal code.