Hibernate Cascade Type SAVE_UPDATE: How to Map Many-to-Many with Extra Fields in User_Group Bridge Table Using JPA Annotations

// User.java (Default Many-to-Many without extra fields)
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
 
    @ManyToMany
    @JoinTable(
        name = "user_group",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "group_id")
    )
    private Set<Group> groups = new HashSet<>();
    // Getters, setters, etc.
}
 
// Group.java (Default Many-to-Many without extra fields)
@Entity
@Table(name = "groups")
public class Group {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
 
    @ManyToMany(mappedBy = "groups")
    private Set<User> users = new HashSet<>();
    // Getters, setters, etc.
}

// UserGroupId.java
import jakarta.persistence.Embeddable;
import java.io.Serializable;
import java.util.Objects;
 
@Embeddable // Marks this as a composite primary key
public class UserGroupId implements Serializable {
 
    private Long userId; // Maps to user.id
    private Long groupId; // Maps to group.id
 
    // Constructors, getters, setters
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        UserGroupId that = (UserGroupId) o;
        return Objects.equals(userId, that.userId) &&
               Objects.equals(groupId, that.groupId);
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(userId, groupId);
    }
}

// UserGroup.java
import jakarta.persistence.*;
import java.time.LocalDateTime;
 
@Entity
@Table(name = "user_group")
public class UserGroup {
 
    @EmbeddedId // Uses the composite key UserGroupId
    private UserGroupId id;
 
    @ManyToOne(fetch = FetchType.LAZY) // Lazy fetch to avoid performance issues
    @MapsId("userId") // Maps userId from UserGroupId to User.id
    @JoinColumn(name = "user_id", nullable = false)
    private User user;
 
    @ManyToOne(fetch = FetchType.LAZY)
    @MapsId("groupId") // Maps groupId from UserGroupId to Group.id
    @JoinColumn(name = "group_id", nullable = false)
    private Group group;
 
    // Extra fields
    @Column(name = "joined_at", nullable = false)
    private LocalDateTime joinedAt;
 
    @Column(name = "role")
    private String role;
 
    // Constructors, getters, setters
 
    // Helper method to set User and Group, and update the composite ID
    public void setUserAndGroup(User user, Group group) {
        this.user = user;
        this.group = group;
        this.id = new UserGroupId(user.getId(), group.getId());
    }
}

// User.java
import jakarta.persistence.*;
import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;
import java.util.HashSet;
import java.util.Set;
 
@Entity
@Table(name = "users")
public class User {
 
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
 
    @Column(nullable = false, unique = true)
    private String username;
 
    // One-to-Many relationship with UserGroup
    @OneToMany(mappedBy = "user", orphanRemoval = true)
    @Cascade(CascadeType.SAVE_UPDATE) // Cascade save/update to UserGroup
    private Set<UserGroup> userGroups = new HashSet<>();
 
    // Helper methods to manage bidirectional relationship
    public void addUserGroup(UserGroup userGroup) {
        userGroups.add(userGroup);
        userGroup.setUser(this);
    }
 
    public void removeUserGroup(UserGroup userGroup) {
        userGroups.remove(userGroup);
        userGroup.setUser(null);
    }
 
    // Getters, setters
}

// Group.java
import jakarta.persistence.*;
import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;
import java.util.HashSet;
import java.util.Set;
 
@Entity
@Table(name = "groups")
public class Group {
 
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
 
    @Column(nullable = false, unique = true)
    private String name;
 
    // One-to-Many relationship with UserGroup
    @OneToMany(mappedBy = "group", orphanRemoval = true)
    @Cascade(CascadeType.SAVE_UPDATE) // Cascade save/update to UserGroup
    private Set<UserGroup> userGroups = new HashSet<>();
 
    // Helper methods to manage bidirectional relationship
    public void addUserGroup(UserGroup userGroup) {
        userGroups.add(userGroup);
        userGroup.setGroup(this);
    }
 
    public void removeUserGroup(UserGroup userGroup) {
        userGroups.remove(userGroup);
        userGroup.setGroup(null);
    }
 
    // Getters, setters
}

// Test Code (e.g., in a JUnit test or main method)
public class Main {
    public static void main(String[] args) {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-persistence-unit");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
 
        // 1. Create a new User
        User user = new User();
        user.setUsername("john_doe");
 
        // 2. Create a new Group
        Group group = new Group();
        group.setName("developers");
        em.persist(group); // Save Group first (optional, depending on cascade)
 
        // 3. Create UserGroup with extra fields
        UserGroup userGroup = new UserGroup();
        userGroup.setJoinedAt(LocalDateTime.now());
        userGroup.setRole("member");
        userGroup.setUserAndGroup(user, group); // Links User, Group, and composite ID
 
        // 4. Associate UserGroup with User and Group
        user.addUserGroup(userGroup);
        group.addUserGroup(userGroup);
 
        // 5. Save User (cascades to UserGroup due to CascadeType.SAVE_UPDATE)
        em.persist(user);
 
        em.getTransaction().commit();
        em.close();
        emf.close();
    }
}