@Entity
@Table(name = "USR")
public class User implements UserDetails {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column(unique = true)
    private String username;    
    private boolean active;

    @ElementCollection(targetClass = Role.class, fetch = FetchType.EAGER)
    @CollectionTable(name = "USR_ROLE", joinColumns = @JoinColumn(name = "USR_ID"))
    @Enumerated(EnumType.STRING)
    private Set<Role> roles;

    @OneToMany(fetch = FetchType.EAGER)
    @JoinColumn(name = "USRGROUPID")
    private Set<UserGroup> userGroups;

@Entity
@Table
public class UserGroup {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String userGroupName;
    private boolean active;

@Entity
@Table(name = "COURSE")
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String coursename;
    private String coursedescription;
    private boolean active;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "COURSETOUSRGROUP",
            joinColumns = {@JoinColumn(name = "USRGROUPID")},
            inverseJoinColumns = {@JoinColumn(name = "COURSEUSRGROUPID")})
    private Set<UserGroup> userGroups = new HashSet<>();

private static final long serialVersionUID = -8706689714326132798L;

serialVersionUID

@GetMapping("/main")
    public String main(Model model,
                       @AuthenticationPrincipal User user) {

@Entity
@Table(name = "USR")
public class User implements UserDetails {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    @Column(unique = true)
    private String username; 
    private boolean active;

    @ElementCollection(targetClass = Role.class, fetch = FetchType.EAGER)
    @CollectionTable(name = "USR_ROLE", joinColumns = @JoinColumn(name = "USR_ID"))
    @Enumerated(EnumType.STRING)
    private Set<Role> roles;

    @ManyToMany(fetch = FetchType.EAGER,
            cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "USR_TO_GROUP",
            joinColumns = @JoinColumn(name = "USRID"),
            inverseJoinColumns = @JoinColumn(name = "GROUPID"))
    private Set<UserGroup> groups;

@Entity
@Table(name = "USR_GROUP")
public class UserGroup implements Serializable {
    private static final long serialVersionUID = -8706689714326132798L;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String userGroupName;
    private boolean active;

    @ManyToMany(mappedBy = "groups")
    private Set<User> users = new HashSet<>();