p:selectOneMenu first ajax request issue - jsf-2

Question

I have a problem with PrimeFaces p:selectOneMenu component when I try to update a p:dataGrid by changing the SelectOneMenu's value.
Xhtml:
<p:selectOneMenu value="#{bussinessOwnersViewerMB.selectedCity}" effect="explode">
<f:selectItems value="#{bussinessOwnersViewerMB.cities}" var="city" itemLabel="#{city.cityName}" itemValue="#{city.cityId}"/>
<p:ajax listener="#{bussinessOwnersViewerMB.handleCityChange}" update="mainform:bolist"/>
</p:selectOneMenu>
<p:dataGrid id="bolist" var="bo" value="#{bussinessOwnersViewerMB.bOwners}" columns="3" rows="#{bussinessOwnersViewerMB.os}">
<p:column>
<p:panel header="#{bo.bOName}">
<h:panelGrid columns="1">
<p:graphicImage value="/resources/images/#{bo.boType}/#{bo.bOId}/mainProfile.jpg"/>
</h:panelGrid>
</p:panel>
</p:column>
</p:dataGrid>
The problem is, when I change the selectOneMenu's value at the first time the datagrid doesn't get updated. What happen is only the postconstructor method is called, after the first change it works fine.
My managed bean is a viewscoped managed bean.
The listener method "bussinessOwnersViewerMB.handleCityChange" is :
public void handleCityChange(ValueChangeEvent event) {
bOwners = bovb.loadAllDistributerByType(new Integer(selectedCity)); // selectedcity is the selected item id
os = Integer.toString(bOwners.size());
}

Answer 1

In the past you couldn't update a dataGrid like that; you had to wrap it in a panelGroup. I don't know wether or not that has changed, but I still do it like that and it still works. So just put your dataGrid inside a panelGroup and update the panelGroup (instead of the dataGrid) in your ajax tag.

Answer 2

In my case the problem was generated by a <p:panel> that contains a <h:panelGroup>, so the partial submit using <p:ajax> did not work. This problem happen when in the form there are empty fields which has required=true.
I updated the <p:panel> in <h:panelGroup>.
From:
<p:panel ... >
...
<h:panelGroup ...>
<!-- Form inside -->
</h:panelGroup>
...
</p:panel>
to
<h:panelGroup ... >
...
<h:panelGroup ...>
<!-- Form inside -->
</h:panelGroup>
...
</h:panelGroup>