Bean-validation constraints not applied in the same phase

Question

I am using

• Myfaces 2.1.12 (with Primefaces 3.5)
• Bean Validation Hibernate Validator 4.3.0.Final
• Lombok 1.12.2
• Omnifaces 1.6.2
• PrettyFaces 3.3.3
• Weld 2.1.0.CR1

to validate a bean (relevant extract below)

@Entity
@Data
public class Controle implements Serializable {
  private static final String FOREIGN_KEY_ID = "CON_ID";

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;
  @Size(max = 256, message = "trop long.")
  @Column(nme = "LIB", length = 256)
  @NotNull
  private String libelle;

  @ManyToMany
  @JoinTable(
    name = "THEME_CONTROLE", 
    joinColumns = @JoinColumn(name = FOREIGN_KEY_ID), 
    inverseJoinColumns = @JoinColumn(name = "THECLE")
  )
  @NotNull(message = "vous devez définir au moins un thème")
  @Size(min = 1, message = "vous devez définir au moins un thème")
  private List<Theme> themes;
  (...)
}

The relevant facelet extract is below

<h:form id="controle">
  (...)
  <p:messages />
  <p:panel>
    (...)
    <h:panelGrid columns="2" columnClasses="label,">
      <p:outputLabel for="libelle" value="Libellé" />
      <p:inputText id="libelle" value="#{controle.controle.libelle}" size="60" />
      (...)
      <p:outputLabel for="themes" value="Thèmes" />
      <p:selectManyButton id="themes" value="#{controle.controle.themes}"
              collectionType="java.util.ArrayList" 
              converter="omnifaces.SelectItemsConverter">
        <f:selectItems value="#{controle.themes}" var="t" itemLabel="#{t.libelle}" itemValue="#{t}" />
      </p:selectManyButton>
      (...)
    </h:panelGrid>
    (...)
    <f:facet name="footer">
      (...)
      <p:commandButton id="saveSubmit" value="#{controle.saveLabel}"
         action="#{controle.save}" icon="ui-icon-disk"
         styleClass="ui-priority-primary" update=":controle" />
    </f:facet>
  </p:panel>
  (...)
</h:form>

The backing-bean is below

@Named(value = "controle")
@ViewScoped
@Data
@Slf4j
@URLMappings(mappings = {
    @URLMapping(id = "controle_create", pattern = ControleController.URL_MAPPING_CONTROLE_CREATE, viewId = ControleController.VIEW_ID),
    @URLMapping(id = "controle", pattern = ControleController.URL_MAPPING_CONTROLE, viewId = ControleController.VIEW_ID)})
public class ControleController implements Serializable {
  (...)
  @URLAction
  public void load() {
    FacesContext.getCurrentInstance().getMessages();
    if (controle == null) {
      if (id != null) {
        controle = controleService.findById(id);
      } else {
        controle = new Controle();
        controle.setAnneeCreation(LocalDate.now().getYear());
      }
      themes = themeService.getAllThemes();
  }
  (...)
  public String save() {
    String destination;
    if (controle.getId() == null) {
        destination = "pretty:controle_create";
    } else {
        destination = "pretty:programme";
    }
    controleService.save(controle);
    return destination;
  }
  (...)
}

In case you are wondering about the collectionType, you can look at “could not initialize proxy - no session” with an open session available

My problem lies with how (or maybe when) validation constraints are validated. More specifically, the @NotNull constraint on the String libelle attribute is working as expected (message is displayed along potential others, saving doesn't occur, ...)

However, it is not the same with the @Size constraint on the List<Theme> themes attribute. With no theme selected (size is 0) and all else validates, nothing seems to happen when I click on save. If I inspect the HTTP response (ajax), I find the validation exception

<?xml version="1.0" encoding="utf-8"?>
<partial-response>
  <error>
    <error-name>org.apache.myfaces.view.facelets.el.ContextAwareELException</error-name>
    <error-message><![CDATA[
      javax.el.ELException: javax.validation.ConstraintViolationException: Validation failed for classes [fr.senat.model.dosleg.Controle] during persist time for groups [javax.validation.groups.Default, ]
      List of constraint violations:[
        ConstraintViolationImpl{
          interpolatedMessage='vous devez définir au moins un thème',
          propertyPath=themes, 
          rootBeanClass=class fr.senat.model.dosleg.Controle, 
          messageTemplate='vous devez définir au moins un thème'
        }
    ]]]></error-message>
  </error>
</partial-response>

Furthermore, if both constraints are not validated, only the first one (libelle is null) is displayed.

This all leads me to think that the @Size constraint on the Collection is only verified during persist phase and not the validation phase. But I don't understand why that would happen.

Thank you in advance for any lead or help with that problem.