Why is hibernate deleting before saveOrUpdate ()?

I have a m: n relation between Column and Element. The entity that owns the relationship is Column. however when I'm saving a map of Columns, the following scenario occurs:

1) In the first iteration, Hibernate:

1.1) saves the first Column element

1.2) inserts the two elements in the column column_element

follows console:

Hibernate: select nextval ('hibernate_sequence')
Hibernate: insert into coluna (COLUNA_ALIAS_OPERACAO, COLUNA_CLAUSULA, COLUNA_DATA_DOIS, COLUNA_DATA_UM, COLUNA_EXIBE_FILTRO, COLUNA_EXIBE_NO_RELATORIO, COLUNA_EXIBE_TOTALIZADOR, COLUNA_INDEX, COLUNA_LABEL, COLUNA_NOME, COLUNA_OPERACAO, COLUNA_OPERACAO_REFERNCIANDO_ALIAS, RELATORIO_ID, COLUNA_TEMPO_DOIS, COLUNA_TEMPO_UM, COLUNA_TIPO_CLAUSULA_TEXTO, COLUNA_TIPO_EXIBICAO, COLUNA_TIPO_FILTRO, COLUNA_ID) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: insert into coluna_elemento (COLUNA_ID, ELEMENTO_DOMINIO_ID) values (?, ?)
Hibernate: insert into coluna_elemento (COLUNA_ID, ELEMENTO_DOMINIO_ID) values (?, ?)

At this point, the column_element of my bank is like this:

coluna_id | elemento_id
    988        860
    988        861

So far everything is happening as expected.

2) In the second iteration, Hibernate:

2.1) saves the second column element

2.2) deleting the column element_element the previously saved element

2.3) inserts the two elements in the column column_element

As the output from the following console shows:

Hibernate: select nextval ('hibernate_sequence')
Hibernate: insert into coluna (COLUNA_ALIAS_OPERACAO, COLUNA_CLAUSULA, COLUNA_DATA_DOIS, COLUNA_DATA_UM, COLUNA_EXIBE_FILTRO, COLUNA_EXIBE_NO_RELATORIO, COLUNA_EXIBE_TOTALIZADOR, COLUNA_INDEX, COLUNA_LABEL, COLUNA_NOME, COLUNA_OPERACAO, COLUNA_OPERACAO_REFERNCIANDO_ALIAS, RELATORIO_ID, COLUNA_TEMPO_DOIS, COLUNA_TEMPO_UM, COLUNA_TIPO_CLAUSULA_TEXTO, COLUNA_TIPO_EXIBICAO, COLUNA_TIPO_FILTRO, COLUNA_ID) values (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate: delete from coluna_elemento where COLUNA_ID=?
Hibernate: insert into coluna_elemento (COLUNA_ID, ELEMENTO_DOMINIO_ID) values (?, ?)
Hibernate: insert into coluna_elemento (COLUNA_ID, ELEMENTO_DOMINIO_ID) values (?, ?)

And so, the column_element element in the database looks like this:

coluna_id  |  elemento_id
   989           860
   989           861

When what I expected was the following:

coluna_id | elemento_id
  988          860
  988          861
  989          860
  989          861

What am I doing wrong?

The code where I save the column map is as follows:

private void salvaColunas() {
    for (Map.Entry<Integer, Coluna> entry : mapaColunas.entrySet()){
        Coluna coluna = entry.getValue();
        coluna.setRelatorio(relatorio);
        colunaDao.saveOrUpdate(coluna);
    }
}

The saveOrUpdate method of my Dao class is as follows:

public void saveOrUpdate(T obj) {
    Session session;
    Transaction tx = null; 
    try {
        session = HibernateUtil.getSessionFactory().getCurrentSession();
    }catch (HibernateException ex) {
        session = HibernateUtil.getSessionFactory().openSession();
    }
    try{
        tx = session.beginTransaction();
        session.saveOrUpdate(obj);
        tx.commit();
      } catch (RuntimeException e) {
          e.printStackTrace();
      } finally {
          if(session.isOpen()) {
              session.close();
          }
      }
 }

The mapping in the column class (already with equals and hashCode methods implemented) looks like this:

@ManyToMany(fetch= FetchType.LAZY, cascade= {CascadeType.PERSIST, CascadeType.REFRESH, CascadeType.MERGE})
@JoinTable(name="coluna_elemento",  joinColumns={
        @JoinColumn(name="COLUNA_ID", nullable=false, updatable=false )},
        inverseJoinColumns = {@JoinColumn(name = "ELEMENTO_DOMINIO_ID", nullable=false, updatable = false)  })
private Set<ElementoDominio> elementosDoDominio;

public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((id == null) ? 0 : id.hashCode());
    result = prime * result + ((label == null) ? 0 : label.hashCode());
    result = prime * result + ((nome == null) ? 0 : nome.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Coluna other = (Coluna) obj;
    if (id == null) {
        if (other.id != null)
            return false;
    } else if (!id.equals(other.id))
        return false;
    if (label == null) {
        if (other.label != null)
            return false;
    } else if (!label.equals(other.label))
        return false;
    if (nome == null) {
        if (other.nome != null)
            return false;
    } else if (!nome.equals(other.nome))
        return false;
    return true;
}

And my Element class (which also has the equals and hashCode methods implemented) looks like this:

@ManyToMany(fetch = FetchType.LAZY, mappedBy = "elementosDoDominio")
private Set<Coluna> colunas;



@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((id == null) ? 0 : id.hashCode());
    result = prime * result + ((label == null) ? 0 : label.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    ElementoDominio other = (ElementoDominio) obj;
    if (id == null) {
        if (other.id != null)
            return false;
    } else if (!id.equals(other.id))
        return false;
    if (label == null) {
        if (other.label != null)
            return false;
    } else if (!label.equals(other.label))
        return false;
    return true;
}

Both the Column and Element classes have more attributes, including the id that is generated by Hibernate. All with getters and setters.

Can someone help me? Thank you