Exception in thread "AWT-EventQueue-0" javax.persistence.PersistenceException: org.hibernate.id.IdentifierGenerationException: ids for this class must be manually assigned before calling save(): presenca.Aluno
        at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:629)
        at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:218)

+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| RA    | int(11)     | NO   | PRI |         |       |
| Nome  | varchar(50) | NO   |     |         |       |
| Email | varchar(80) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

package presenca;

import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.Table;

/**
* Entity class Aluno
*
* @author Bruno
*/
@Entity
@Table(name = "aluno")
@NamedQueries( {
        @NamedQuery(name = "Aluno.findByRa", query = "SELECT a FROM Aluno a WHERE a.ra = :ra"),
        @NamedQuery(name = "Aluno.findByNome", query = "SELECT a FROM Aluno a WHERE a.nome = :nome"),
        @NamedQuery(name = "Aluno.findByEmail", query = "SELECT a FROM Aluno a WHERE a.email = :email")
    })
public class Aluno implements Serializable {

    @Id
    @Column(name = "RA", nullable = false, insertable = true)
    private Integer ra;

    @Column(name = "Nome", nullable = false)
    private String nome;

    @Column(name = "Email")
    private String email;
   
    /** Creates a new instance of Aluno */
    public Aluno() {
    }

    /**
     * Creates a new instance of Aluno with the specified values.
     * @param ra the ra of the Aluno
     */
    public Aluno(Integer ra) {
        this.ra = ra;
    }

    /**
     * Creates a new instance of Aluno with the specified values.
     * @param ra the ra of the Aluno
     * @param nome the nome of the Aluno
     */
    public Aluno(Integer ra, String nome) {
        this.ra = ra;
        this.nome = nome;
    }

    /**
     * Gets the ra of this Aluno.
     * @return the ra
     */
    public Integer getRa() {
        return this.ra;
    }

    /**
     * Sets the ra of this Aluno to the specified value.
     * @param ra the new ra
     */
    public void setRa(Integer ra) {
        this.ra = ra;
    }

    /**
     * Gets the nome of this Aluno.
     * @return the nome
     */
    public String getNome() {
        return this.nome;
    }

    /**
     * Sets the nome of this Aluno to the specified value.
     * @param nome the new nome
     */
    public void setNome(String nome) {
        this.nome = nome;
    }

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

    /**
     * Sets the email of this Aluno to the specified value.
     * @param email the new email
     */
    public void setEmail(String email) {
        this.email = email;
    }

    /**
     * Returns a hash code value for the object.  This implementation computes
     * a hash code value based on the id fields in this object.
     * @return a hash code value for this object.
     */
    @Override
    public int hashCode() {
        int hash = 0;
        hash += (this.ra != null ? this.ra.hashCode() : 0);
        return hash;
    }

    /**
     * Determines whether another object is equal to this Aluno.  The result is
     * <code>true</code> if and only if the argument is not null and is a Aluno object that
     * has the same id field values as this object.
     * @param object the reference object with which to compare
     * @return <code>true</code> if this object is the same as the argument;
     * <code>false</code> otherwise.
     */
    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Aluno)) {
            return false;
        }
        Aluno other = (Aluno)object;
        if (this.ra != other.ra && (this.ra == null || !this.ra.equals(other.ra))) return false;
        return true;
    }

    /**
     * Returns a string representation of the object.  This implementation constructs
     * that representation based on the id fields.
     * @return a string representation of the object.
     */
    @Override
    public String toString() {
        return "presenca.Aluno[ra=" + ra + "]";
    }
   
}

    private void criaNovoAluno() {
        initEntityManager();
       
        em.getTransaction().begin();
        Aluno novoAluno = new Aluno();
        novoAluno.setRa(Integer.getInteger(jTextFieldRA.getText()));
        novoAluno.setNome(jTextFieldNome.getText());
        novoAluno.setEmail(jTextFieldEmail.getText());
        em.persist(novoAluno); // THIS LINE THROWS THE EXCEPTION
        em.getTransaction().commit();
    }