填充依赖的JSF菜单无法正常工作

Question

我有一个HashMap availableGovernates持有密钥（id）和值为governate个对象 我试图根据选择的政府填充城市菜单，当我检查时，传递给函数form.governate的（getGovAvaiableCities）的值为空，但在提交后，form.governate已设置

<h:selectOneMenu id="governates" value="#{form.governate}"
            converter="converter">
            <f:selectItems value="#{data.availableGovernates.entrySet()}"
                var="governate" itemValue="#{governate.value}"
                itemLabel="#{governate.value.name}"></f:selectItems>
            <f:ajax execute="governates" render="cities" />
        </h:selectOneMenu>

        <h:selectOneMenu id="cities" value="#{form.city}"
            converter="converter">
            <f:selectItems
                value="#{form.getGovAvaiableCities(form.governate).entrySet()}" var="city" 
                itemValue="#{city.value}" itemLabel="#{city.value.name}"></f:selectItems>

        </h:selectOneMenu>