JPA ManyToMany form

I have a problem with JEE projet, I have one ManyToMany relation but I dont know how to make it work in the form.

I have a Tache class with:

@Entity
public class Tache implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String nom;
    @ManyToMany(fetch = FetchType.EAGER)
    private List<Developpeur> lesDev;

    public int getId() {return id;}
    public void setId(int id) {this.id = id;}
    public String getNom() {return nom;}
    public void setNom(String nom) {this.nom = nom;}
    public List<Developpeur> getLesDev() {return lesDev;}
    public void setLesDev(List<Developpeur> lesDev) {this.lesDev = lesDev;}
}

A Developpeur class with:

@Entity
public class Developpeur implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String prenom;    
    private String nom;
    @ManyToMany(mappedBy = "lesDev")
    private List<Tache> lesTache;

    public int getId() {return id;}
    public void setId(int id) {this.id = id;}
    public String getPrenom() {return prenom;}
    public void setPrenom(String prenom) {this.prenom = prenom;}
    public String getNom() {return nom;}
    public void setNom(String nom) {this.nom = nom;}
    public List<Tache> getLesTache() {return lesTache;}
    public void setLesTache(List<Tache> lesTache) {this.lesTache = lesTache;}

TacheBean is the controller:

@Named
@RequestScoped
public class TacheBean {
    private Tache uneTa = new Tache();
    private List<Tache> lesTa;
    @PersistenceContext
    private EntityManager em;
    @Resource
    private UserTransaction tx;

    public String validerEditer()
    {
        try{
        tx.begin();
        em.merge(uneTa);
        tx.commit();
        FacesMessage message = new FacesMessage("Tache éditée avec succès");
        FacesContext.getCurrentInstance().addMessage(null, message);
        } catch (Exception ex) {
            Logger.getLogger(TacheBean.class.getName()).log(Level.SEVERE, null, ex);
        } 
        return "tache";
    }

And the view for the form:

<h:form>
        <p:growl id="message_erreur" life="8000"/>
        <div class="formulaire">
            <h:inputHidden value="#{tacheBean.uneTa.id}"/>
            <div class="titreformulaire">Editer/Ajouter tache <span class="alerte">Les champs notés * sont obligatoires</span></div>

            <p class="label"><label>Nom *</label></p>
            <p><p:inputText label="Nom" id="nom" value="#{tacheBean.uneTa.nom}" required="true" /></p>


            <p><label>Developpeur(s) *</label></p>
            <p><p:selectManyMenu id="basic" value="#{tacheBean.uneTa.lesDev}">                        
                    <f:selectItems value="#{developpeurBean.readAll()}" var="d" itemValue="#{d.id}" itemLabel="#{d}"/>
                </p:selectManyMenu>
            </p>


            <p><p:commandButton value="Valider" action="#{tacheBean.validerEditer()}" styleClass="boutonvalider" ajax="false" update="message_erreur"/></p>
        </div>
    </h:form>

I am sure that the value of selectManyMenu is bad... I think I need to feed the developpeur list before the merge but I dont see how to do this :/ What should I do in the form and in the controller for it to work?