下面的java spring引导代码表示对java后端的一个简单rest调用，该调用在mysql数据库上执行插入操作，但是当它执行rest调用时，我报告了下面的错误，我无法定义在mysql数据库中插入时的问题在何处，如何解决控制台中hibernate表示的错误？谢谢您
错误消息休眠：

"could not execute statement; SQL [n/a]; constraint [null]; 
nested exception is org.hibernate.exception.ConstraintViolationException: 
could not execute statement

控制器：

@PostMapping("/newuser")
    public User createUser(@Valid @RequestBody User userDetails) {
        System.out.println("Sono nella creazione dell'utente");
        return userRepository.save(userDetails);
    }

型号：

@Entity
@Table(name = "users")
@EntityListeners(AuditingEntityListener.class)
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", unique = true, nullable = false)
    private long id;

    @Column(name = "first_name", nullable = false)
    private String firstName;

    @Column(name = "last_name", nullable = false)
    private String lastName;

    @Column(name = "email_address", nullable = true)
    private String email;

    /**
     * Gets id.
     *
     * @return the id
     */
    public long getId() {
        return id;
    }

    /**
     * Sets id.
     *
     * @param id the id
     */
    public void setId(long id) {
        this.id = id;
    }

    /**
     * Gets first name.
     *
     * @return the first name
     */
    public String getFirstName() {
        return firstName;
    }

    /**
     * Sets first name.
     *
     * @param firstName the first name
     */
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    /**
     * Gets last name.
     *
     * @return the last name
     */
    public String getLastName() {
        return lastName;
    }

    /**
     * Sets last name.
     *
     * @param lastName the last name
     */
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    /**
     * Gets email.
     *
     * @return the email
     */
    public String getEmail() {
        return email;
    }

    /**
     * Sets email.
     *
     * @param email the email
     */
    public void setEmail(String email) {
        this.email = email;
    }

    /**
     * Gets created at.
     *
     * @return the created at
     */

    /**
     * Sets created at.
     *
     * @param createdAt the created at
     */

    /**
     * Gets created by.
     *
     * @return the created by
     */

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", email='" + email + '\'' +
                '}';
    }

}

存储库

import xxx.mode.User;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

/**
 * The interface User repository.
 *
 * @author xxxx
 */
@Repository
public interface UserRepository extends JpaRepository<User, Long> {}