JPA: ManyToMany issue - Spring Boot Project - spring-boot

Question

I want to establish many-to-many relation between two table. One is called User and the other is Tag.
My goal is to add previously created Tag lists to newly created User objects. Tag must be added to the database first, and then merged with User after selecting from existing ones.
How can I solve this problem? Where am I doing wrong? Any advice?
Thank you.
All my codes are as follows.
Filename = Demo2Application.java
#SpringBootApplication
public class Demo2Application implements CommandLineRunner {
#Autowired
private UserService userService;
#Autowired
private TagService tagService;
public static void main(String[] args) {
SpringApplication.run(Demo2Application.class, args);
}
#Override
public void run(String... args) throws Exception {
//Scenario: Tag list is already created.
Tag tag1 = new Tag("strong", true); //id = 1
Tag tag2 = new Tag("weak", true); //id = 2
Tag tag3 = new Tag("nice", true); //id = 3
Tag tag4 = new Tag("clever", true); //id = 4
tagService.save(tag1);
tagService.save(tag2);
tagService.save(tag3);
tagService.save(tag4);
//Scenario: Defining a new user.
User user1 = new User("foo", "foo#gmail.com");
//Scenario: Appropriate predefined tags are being added to the new user object.
user1.addTag(tagService.findById(1));
user1.addTag(tagService.findById(4));
//Scenario: Registering a User object to the database.
userService.save(user1);
}
}
Filename = entity/Model.java
#MappedSuperclass
public class Model {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private long id;
#CreationTimestamp
private Date createdAt;
#UpdateTimestamp
private Date updatedAt;
// standard constructors, getters, and setters
}
Filename = entity/User.java
#Entity
#Table(name = "user")
public class User extends Model {
#Column(name = "name")
private String name;
#Column(name = "email")
private String email;
#ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
#JoinTable(name = "user_tag",
joinColumns = {#JoinColumn(name = "user_id")},
inverseJoinColumns = {#JoinColumn(name = "tag_id")})
private List<Tag> tags = new ArrayList<>();
//
public void addTag(Tag tag) {
tags.add(tag);
}
// standard constructors, getters, and setters
Filename = entity/Tag.java
#Entity
#Table(name = "tag")
public class Tag extends Model {
#Column(name = "tag_name")
private String tagName;
#Column(name = "tag_active")
private boolean tagActive;
#ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
#JoinTable(name = "user_tag",
joinColumns = {#JoinColumn(name = "tag_id")},
inverseJoinColumns = {#JoinColumn(name = "user_id")})
private List<User> users = new ArrayList<>();
// standard constructors, getters, and setters
Repositories = TagRepository, UserRepository
public interface UserRepository extends JpaRepository<User, Long> {
}
public interface TagRepository extends JpaRepository<Tag, Long> {
}
Filename = service/UserService.java
#Service
public class UserService {
private UserRepository userRepository;
#Autowired
public UserService(UserRepository userRepository) {
this.userRepository = userRepository;
}
public User findById(long id) {
Optional<User> result = userRepository.findById(id);
return result.orElse(null);
}
public void save(User user) {
userRepository.save(user);
}
}
Filename = service/TagService.java
#Service
public class TagService {
private TagRepository tagRepository;
#Autowired
public TagService(TagRepository tagRepository) {
this.tagRepository = tagRepository;
}
public Tag findById(long id) {
Optional<Tag> result = tagRepository.findById(id);
return result.orElse(null);
}
public void save(Tag tag) {
tagRepository.save(tag);
}
}
Filename = resources/application.properties
server.port=7070
## Database (PostgreSQL + Hikari + JPA)
spring.datasource.type=com.zaxxer.hikari.HikariDataSource
spring.datasource.hikari.connectionTimeout=20000
spring.datasource.hikari.maximumPoolSize=5
spring.datasource.hikari.minimum-idle=5
spring.datasource.hikari.idle-timeout=300000
spring.datasource.hikari.max-lifetime=1200000
#
spring.datasource.url=jdbc:postgresql://localhost:5432/demo
spring.datasource.username=admin
spring.datasource.password=1234
#
spring.jpa.hibernate.ddl-auto=create-drop
## Database ##