Omit validation when making change on selectOneMenu with ajax - jsf

Question

I have a selectOneMenu in a form with an ajax to update a separate component when it is triggered. This menu also uses validation. This first item in the menu is null and the validation makes sure that you can't submit the form if this item is null. However, I do want the user to be able to select the null item and that it set the value of that item in my backing bean to null. However the set is not happening when the user selects the null item because the validation failure occurs first and is throwing an exception preventing the value from being set to null in my backing bean.
Here is my code
<h:form>
...
<p:selectOneMenu value="#{myBean.container}">
<f:selectItem itemLabel="Select One" itemValue=#{null} />
<f:selectItems value="#{myBean.getContainerItems()}" />
<p:ajax event="change" update="containerNumSection" />
<f:validateRequired />
</p:selectOneMenu>
...
</h:form>
Is there any way that I can have the value in my backing bean set to null when the user selects the null item while keeping the validation?

Answer 1

I have found a somewhat dirty workaround using a custom validation method that is able to distinct cases where either the selectOneMany is changed or the submit button is pressed:
<h:form>
<p:selectOneMenu id="select" value="#{myBean.stringVal}"
validator="#{myBean.validateSelection}">
<f:selectItem itemLabel="nothing" itemValue="#{null}" />
<f:selectItem itemLabel="a" itemValue="a" />
<f:selectItem itemLabel="b" itemValue="b" />
<p:ajax update="#form" process="#this"/>
</p:selectOneMenu>
<p:commandButton value="submit" update="#form" process="#form">
<f:param name="selectionRequired" value="true"/>
</p:commandButton>
<h:message for="select" />
<h:outputText value="#{myBean.stringVal}" />
</h:form>
If the user presses the submit button, a request parameter named "selectionRequired" is set to "true". If this is the case, the validation method delegates to the javax.faces.validator.RequiredValidator.validate() method, else it simply does nothing.
MyBean.validateSelection:
public void validateSelection(FacesContext context, UIComponent component, Object value) {
String selectionRequired = FacesContext.getCurrentInstance().getExternalContext()
.getRequestParameterMap()
.get("selectionRequired");
if (Boolean.valueOf(selectionRequired)) {
new RequiredValidator().validate(context, component, value);
}
}
I had to enable javax.faces.VALIDATE_EMPTY_FIELDS in order for this to work, else the validateSelection method was not invoked at all when #{null} is selected.
web.xml:
<context-param>
<param-name>javax.faces.VALIDATE_EMPTY_FIELDS</param-name>
<param-value>true</param-value>
</context-param>