Application with bug which will be fixed by ChatGPT.

DemoApplication.java

package com.example.demo;

import java.io.Serializable;
import java.util.Arrays;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Service;

@SpringBootApplication
public class DemoApplication {

	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}

	@Bean
	ApplicationRunner runner(DemoService demoService) {
		return args -> {
			var demoObj = new DemoObj();
			demoObj.setName("Giu");
			var demoObjPk = new DemoObjPk();
			demoObjPk.setSeq(1);
			demoObjPk.setOutraSeq(1);
			demoObj.setId(demoObjPk);

			var parentObj = new ParentDemoObj();
			parentObj.setId(1);
			parentObj.setDemoObjs(Arrays.asList(demoObj));
			System.out.println(demoService.create(parentObj));
		};
	}

}

@Entity
class ParentDemoObj {
	@Id
	Integer id;

	@OneToMany(mappedBy = "id.seq", cascade = CascadeType.PERSIST)
	List<DemoObj> demoObjs;

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public List<DemoObj> getDemoObjs() {
		return demoObjs;
	}

	public void setDemoObjs(List<DemoObj> demoObjs) {
		this.demoObjs = demoObjs;
	}

	@Override
	public String toString() {
		return "ParentDemoObj [id=" + id + ", demoObjs=" + demoObjs + "]";
	}

}

@Entity
class DemoObj {
	@EmbeddedId
	public DemoObjPk id;
	public String name;

	public DemoObj() {
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public DemoObjPk getId() {
		return id;
	}

	public void setId(DemoObjPk id) {
		this.id = id;
	}

	@Override
	public String toString() {
		return "DemoObj [id=" + id + ", name=" + name + "]";
	}
}

@Embeddable
class DemoObjPk implements Serializable {
	Integer seq;
	Integer outraSeq;

	public Integer getSeq() {
		return seq;
	}

	public void setSeq(Integer seq) {
		this.seq = seq;
	}

	public Integer getOutraSeq() {
		return outraSeq;
	}

	public void setOutraSeq(Integer outraSeq) {
		this.outraSeq = outraSeq;
	}

	@Override
	public String toString() {
		return "DemoObjPk [seq=" + seq + ", outraSeq=" + outraSeq + "]";
	}

}

@Service
class DemoService {
	private DemoRepository repository;

	public DemoService(DemoRepository repository) {
		this.repository = repository;
	}

	public ParentDemoObj create(ParentDemoObj demoObj) {
		return repository.save(demoObj);
	}

}

interface DemoRepository extends JpaRepository<ParentDemoObj, Integer> {

}