primefaces p:rowEdit cascading selectoneMenu - jsf-2

Question

I have an Office datatable that displays department name and role name. department and role are objects. when I select rowedit, when there is a change in the department, the roles should be reloaded. I am not sure how to pass the selected department and rerender the roles.
<p:column headerText="Department">
<p:cellEditor>
<f:facet name="output"><h:outputText value="#{office.department.name}" /></f:facet>
<f:facet name="input">
<p:selectOneMenu value="#{office.department}" converter="#{departmentConverter}" style="width:96%" >
<f:selectItems value="#{departments}" var="dept" itemLabel="#{dept.name}" itemValue="#{dept}"/>
<p:ajax event="change" listener="#{officeManagmentController.onDepartmentChanged()}"/>
</p:selectOneMenu>
</f:facet>
</p:cellEditor>
</p:column>
<p:column headerText="Role">
<p:cellEditor>
<f:facet name="output"><h:outputText value="#{office.role.name}" /></f:facet>
<f:facet name="input">
<p:selectOneMenu value="#{office.role}" converter="#{roleConverter}" style="width:96%" >
<f:selectItems value="#{officeManagementController.assignableRoles}" var="role" itemLabel="#{role.name}" itemValue="#{role}"/>
</p:selectOneMenu>
</f:facet>
</p:cellEditor>
</p:column>
the following is my officecontroller managedbean
#ConversationScoped
public class OfficeManagementController implements Serializable {
public void onRowSelect(SelectEvent event) {
FacesMessage msg = new FacesMessage(selectedOffice.getName()+" selected");
selectedDepartment=selectedOffice.getDepartment();
FacesContext.getCurrentInstance().addMessage(null, msg);
}
public void onDepartmentChanged() {
if(selectedOffice!=null)selectedSystem=selectedOffice.getDepartment();
log.info("Calling onDepartmentChanged " + ((selectedDepartment != null) ? selectedDepartment.getName() : "null"));
if(selectedDepartment != null) {
assignableRoles = roleRepo.findByDepartment(selectedDepartment);
for(int i=0;i<assignableRoles.size();i++){
System.out.println(assignableRoles.get(i));
}
} else {
assignableRoles=null;
}
}
}
What I noticed is the selectedOffice is null in the onDepartmentChanged() method, but in the onrowselect, the value is not null. I haven't progressed more than that. I have to dynamically change the roles when a department changes. Why is the selectedOffice null?