JPA Nested object persistence best practice

Question

I have a JPA Entity where I need to reference a list of nested object references.

I do not want a separate Entity and separate table for this nested object to not pollute my DB schema.

@ElementCollection only work with serializable types so this is one is out. @Embeddable wouldn't work with List.

The object that I would like to store is as follows:

Object {
    private UUID id;
    private TypeEnum type;
    private SecondTypeEnum secondType;
}

What options do I have?

Tried with @ElementCollection, separate Entity and @Embeddable

Answer 1

How to save list of objects in database field

The idea is to convert a list of objects to string. SpringBoot uses jackson library for manipulating json https://github.com/FasterXML/jackson. There is ObjectMapper class in this library that can convert Java objects to String and vice versa. Note that list of objects is also an Object in Java.

Minimal reproducible example

Service class

@Service
public class MyService {

    @Autowired
    MyRepository myRepository;

    @Autowired
    ObjectMapper objectMapper;

    public MyEntity saveObjects(List<MyObject> objects) throws JsonProcessingException {
        String json = objectMapper.writeValueAsString(objects); // convert list of objects to string
        MyEntity myEntity = new MyEntity(); // create new entity
        myEntity.id = UUID.randomUUID(); // set id
        myEntity.objects = json; // set objects
        return myRepository.save(myEntity); // save entity in database
    }

    public List<MyObject> readObjects(UUID id) throws JsonProcessingException {
        System.out.println("reading id " + id);
        MyEntity myEntity = myRepository.findById(id).orElseThrow(); // read entity from database
        String json = myEntity.objects; // get field objects as string
        System.out.println(json); // print json as string
        return objectMapper.readValue(json,
                                      new TypeReference<List<MyObject>>() {}
        ); // convert json string to list of objects
    }

}

Other classes used in example

public class MyObject {
    public UUID id;
    // other fields here - type, second type etc
}

@Entity
@Table(name = "example")
public class MyEntity {
    @Id
    public UUID id;
    public String objects; // list of MyObjects as json
}

public interface MyRepository extends JpaRepository<MyEntity, UUID> {
}

Test to proof concept

@SpringBootTest
public class MyServiceIT {

    @Autowired
    MyService service;


    @Test
    void testSavingAndReading_positive() throws JsonProcessingException {
        // prepare data
        UUID uuid = UUID.randomUUID();
        MyObject object1 = new MyObject();
        object1.id = uuid;
        MyObject object2 = new MyObject();
        object2.id = uuid;
        List<MyObject> objects = List.of(object1, object2);
        // save data in database
        MyEntity savedEntity = service.saveObjects(objects);
        // read back
        List<MyObject> result = service.readObjects(savedEntity.id);
        // verify
        assertEquals(2, result.size());
        assertEquals(uuid, result.get(0).id);
        assertEquals(uuid, result.get(1).id);
    }

}

For reproducing this example you have to create a table in underlying database as

create table example (id uuid, objects text);

and provide database spec in application.yml like

spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/postgres
    username: postgres
    password: ss