Primefaces selectone menu doesn't update value

I have an issue with Primefaces Select One Menu, I used it with a Java List. Here's my code:

<h:form id="frmRatePlanDetail">            
        <h:panelGrid columns="2">       
            <p:outputLabel value="Rate plan detail name" for="txtRatePlanDetailName"/>                            
            <p:inputText id="txtRatePlanDetailName"
                         required="true"
                         maxlength="400"
                         value="#{ratePlanDetailController.ratePlanDetail.ratePlanDetailName}"
                         style="width: 300px;"/>                                                           
            <c:forEach begin="0" end="#{ratePlanDetailController.listMappingDetail.size()-1}"
                       var="idx" step="1" >                    
                <p:outputLabel value='#{ratePlanDetailController.listMappingDetail.get(idx).propertyId.propertyName}' for="cboProperty"/>                    
                <p:selectOneMenu
                    id="cboProperty"
                    required="true"
                    style="min-width: 306px;"
                    value="#{ratePlanDetailController.listMappingDetail.get(idx).zoneId.zoneId}">
                    <f:selectItems value="#{ratePlanDetailController.listZoneByZoneMapId(ratePlanDetailController.listMappingDetail.get(idx).zoneId.zoneId)}"
                                   var="z"
                                   itemLabel="#{z.zoneName}"
                                   itemValue="#{z.zoneId}"/>
                    <p:ajax event="change" update="@this" />
                </p:selectOneMenu>
            </c:forEach>
        </h:panelGrid>
        <p:spacer height="20"/>
        <div align="right" style="margin-top: 5px;">
            <p:column colspan="2" >
                <p:commandButton
                    value="OK" 
                    process="frmRatePlanDetail"
                    action="#{ratePlanDetailController.insert()}"
                    update="@([id$=treeProfileEvent]) @([id$=frmRatePlanDetail])" />
                <p:commandButton value="#{lang['common.button.cancel']}"
                                 immediate="true"
                                 onclick="PF('wdlgRatePlanDetail').hide()"
                                 update="@([id$=treeProfileEvent])" />
            </p:column>
        </div>
    </h:form>

ratePlanDetailController.listMappingDetail.get(idx).zoneId.zoneId is an Interger.

It look like select one menu doesn't update value when I clicked commandButton 'OK'.

I always get null value in ratePlanDetailController.listMappingDetail.get(idx).zoneId.zoneId.

Is there anyway to update the value for an object in List ?Any help would be great