@Entity
@Table(name = "operations")
public class Operations implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private Integer id;
    @Column(name = "name")
    private String name;
    @Column(name = "description")
    private String description;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "operation")
    private List<Operjrnl> operjrnlList;
.........
}

@Entity
@Table(name = "users")
public class Users implements Serializable {
    @Id
    @Column(name = "LOGIN")
    private String login;
    @Column(name = "PASSHASH")
    private String passhash;
    @JoinColumn(name = "STAFF", referencedColumnName = "PERSON_ID")
    @ManyToOne(optional = false)
    private Staff staff;
    @JoinColumn(name = "ROLE_ID", referencedColumnName = "ROLE_ID")
    @ManyToOne(optional = false)
    private Roles roleId;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "user")
    private List<Operjrnl> operjrnlList;
..................
}

selectedUser.getOperjrnlList().clear(); //Чистим список операций по пользователю
um.edit(selectedUser);

selectedUser.getOperjrnlList().add(operation);
um.edit(selectedUser);