CRUD API Spring Boot Make a Copy of an entry

I am trying to make a copy of an instance which I am fetching from a CRUD Repository. I want to store the copy of an instance but with a different primary key. In the copy method which I have made in the service class, when I try to make a copy, it throws an error saying org.hibernate.HibernateException: identifier of an instance of SpringBootStarter.Topic.Topic was altered from <id> to <new_id> When I hit a GET request on postman after making the copy, I want to see both the original and the copy in the result (but the copy with a different primary key.)

Can somebody please help me?

import javax.persistence.Entity;
import javax.persistence.Id;
@Entity
public class Topic {
@Id
private String id;
private String name;
private String description;
public Topic(){
}
public Topic(String id, String name, String description) {
this.id = id;
this.name = name;
this.description = description;
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
}

Below is the Controller Class

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;
import java.util.List;
@RestController
public class TopicController {
@Autowired
private TopicService topicService;
@GetMapping("/topics")
public List<Topic> getAllTopics(){
return topicService.getAllTopics();
}
@GetMapping("/topics/{id}")
public Topic getTopic(@PathVariable String id){
return topicService.getTopic(id);
}
@PostMapping("/topics")
public void addTopic(@RequestBody Topic topic){
topicService.addTopic(topic);
}
@PostMapping("topics/{id}/{new_id}")
public void copyTopic(@PathVariable String id, @PathVariable String new_id){
topicService.copyTopic(id, new_id); }
@PutMapping("/topics/{id}")
public void updateTopic(@RequestBody Topic topic, @PathVariable String id){
topicService.updateTopic(topic, id);
}
@DeleteMapping("/topics/{id}")
public void deleteTopic(@PathVariable String id){
topicService.deleteTopic(id);
}
}

Below is the Service Class

package SpringBootStarter.Topic;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;
@Service
public class TopicService {
@Autowired
private TopicRepository topicRepository;
public List<Topic> getAllTopics(){
List<Topic> topics = new ArrayList<>();
topicRepository.findAll().forEach(topics :: add);
return topics;
}
public Topic getTopic(String id){
Optional<Topic> optional = topicRepository.findById(id);
return optional.get();
}
public void addTopic(Topic topic){
topicRepository.save(topic);
}
public void copyTopic(String id, String new_id){
Topic topic = topicRepository.findById(id).get();
Topic topicCopy = topic;
topicCopy.setId(new_id);
addTopic(topicCopy);
}
public void updateTopic(Topic topic, String id){
topicRepository.save(topic);
}
public void deleteTopic(String id){
topicRepository.deleteById(id);
}
}

Below is the Topic Repository

import org.springframework.data.repository.CrudRepository;
public interface TopicRepository extends CrudRepository<Topic, String> {
}

The persistence context holds the lifecycle of all entities. When you fetch an entity it will be an attached entity within that transsaction. Because the reference of your object does not change, the persistence context will know that it's still the same object in the database which does not allow it's identifier to change.
If you want to create a new entry, you will have to create a new object using the new keyword and persist that one.

So instead of changing the identifier like so

public void copyTopic(String id, String new_id){
Topic topic = topicRepository.findById(id).get();
Topic topicCopy = topic;
topicCopy.setId(new_id);
addTopic(topicCopy);
}

Create a new Topic entity and persist it like so

public void copyTopic(String id, String new_id){
Topic topic = topicRepository.findById(id).get();
Topic topicCopy = new Topic(topic);
topicCopy.setId(new_id);
addTopic(topicCopy);
}

My advice is to read up on the basics of Hibernate because there are a lot of pitfalls when using an ORM. It's never a good idea to start using one without understanding the very basics.