How do I insert data into 2 tables if I have just one registration form

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Java

Spring

I have 2 tables:

<pre lang="java">
public class User implements Serializable {
	private static final int MIN_SIZE = 3;
	private static final int MAX_SIZE = 50;

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO)
	private Long id;

	@NotEmpty
	@Column(name = "first_name", nullable = false)
	@Size(min = MIN_SIZE, max = MAX_SIZE)
	private String firstName;

	@NotEmpty
	@Column(name = "last_name", nullable = false)
	@Size(min = MIN_SIZE, max = MAX_SIZE)
	private String lastName;

	@Email
	@Column(name = "email")
	private String email;

	@NotEmpty
	@Column(name = "password")
	private String password;

	@Column(name = "confirm_password")
	private String confirmPassword;

	@Column(name = "year_degree")
	private int yearDegree;

	@Column(name = "degree")
	private String degree;

	@Column(name = "domain")
	private String domain;

	@OneToMany(mappedBy = "user", fetch = FetchType.EAGER)
	private Set<Theme> themeList = new TreeSet<Theme>();

	@OneToMany(mappedBy = "user", fetch = FetchType.EAGER)
	private Set<Enroll> enrollList = new TreeSet<Enroll>();

	@OneToMany(mappedBy = "user", fetch = FetchType.EAGER)
	private Set<Role> userRoles = new TreeSet<Role>();
//setters and getters

and

public class Role implements Serializable{
	
	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	private Long id;
	
	@Column(name="userRole")
	private String userRole="ROLE_STUDENT";
	
	@ManyToOne
	@JoinColumn(name="user_id")
	private User user;
//setters and getters

Each user can be a student or a teacher.

And a register form:

<sf:form class="registerForm" method="POST" modelAttribute="user">
      <div class="top-margin form-group">
          <label>Register as a</label> 
          <sf:select id="input_loginas" name="input_loginas" class="form-control">
            <option value="">Choose an option...</option>
            <option value="student">student</option>
            <option value="teacher">teacher</option>
       	</sf:select>
     </div>
     <div class="top-margin form-group">
         <label>First Name</label>
         <sf:input path="firstName" type="text" name="firstName" id="firstName" class="form-control">
         </sf:input>
     </div>

The roles should be student or teacher.
If an user selects an option from the dropdown list, I want to insert in role table his role and in the user table the rest of the details and I don't know how because are 2 tables.(I want to use these roles in spring security)