http://localhost:8080/groups

{
  "name": "Group 1",
  "user": "http://localhost:8080/users/1"
}

@Column(name="user_group_id")
@ManyToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@JoinTable(name = "app_user__user_group",
        joinColumns = {
                @JoinColumn(name = "app_user_id", nullable = false, updatable = false) },
        inverseJoinColumns = {
                @JoinColumn(name = "user_group_id", nullable = false, updatable = false)})
private Set<UserGroup> groups;

@Column(name = "app_user_id")
@ManyToMany(fetch = FetchType.LAZY, mappedBy = "groups")
private Set<AppUser> users;