cannot set managed-property parameter with a JSF expression value - jsf

Question

I have a strange problem which I have been trying to fix for some time but am stuck in one place and don't quite understand what's going on here.
My index page looks like this:
<h:selectOneMenu id="selectMenu" value="#{indexBean.model.selected_id}" styleClass="indexItems">
<f:selectItems value="#{indexBean.myModelValues}" />
<a4j:support event="onchange" reRender="peek" />
</h:selectOneMenu>
<br>
<h:outputText id="peek" value ="#{indexBean.model.selected_id}"/>
<br>
<a4j:commandButton value="Go to Form" action="form" styleClass="indexItems">
<f:param name="selected" value="#{indexBean.model.selected_id}" />
</a4j:commandButton>
The commandButton sends the user to a next page when I want to get the selected position from the selectOneMenu. The problem is that nothing is sent. When I select some value from the Menu the 'peek' outputText is rerendered properly and I can see the correct selection. However it is not sent to the next page and Bean. Surprisingly when I change the value of the parameter to a fixed String it works!!! So i.e. this:
<f:param name="selected" value="someValue1" />
Is read correctly in the next Bean!!! All of the beans are requested scoped with RichFaces #KeepAlive annotation (I tried without the annotation and it's the same). Ihave treied:
-changing a4j:commandButton to h:commandLink or h:outputLink
-changing indexBean.model.selected_id to indexBean.selected_id
-finally changing the input source from h:selectOneMenu to h:inputText
Nothing helps - it is still the same - a hardcoded String is passed correctly to the next page, but when I try to use the expresion the parameter is always empty.
Here I post a snippet of the faces-config.xml:
<managed-bean>
<managed-bean-name>indexBean</managed-bean-name>
<managed-bean-class>id.webapp.beans.IndexBean</managed-bean-class>
<managed-bean-scope>request</managed-bean-scope>
</managed-bean>
<managed-bean>
<managed-bean-name>model</managed-bean-name>
<managed-bean-class>id.webapp.beans.Model</managed-bean-class>
<managed-bean-scope>request</managed-bean-scope>
</managed-bean>
<managed-bean>
<managed-bean-name>formBean</managed-bean-name>
<managed-bean-class>id.webapp.beans.FormBean</managed-bean-class>
<managed-bean-scope>request</managed-bean-scope>
<managed-property>
<property-name>selectedValue</property-name>
<value>#{param.selected}</value>
</managed-property>
</managed-bean>
Does any one have an idea why this doesn't work? I have used the managed-properties parameters like that in the past and it worked (meaning using some dynamically changed values not a static String).

Answer 1

The <f:param> is evaluated during the initial request (when the form is to be displayed), not during the form submit request. So it still remains the initial value, not the changed value.
Give the <a4j:commandButton> an id and refer it in the reRender as well so that its <f:param> get re-evaluated before you press the button.
<h:selectOneMenu id="selectMenu" value="#{indexBean.model.selected_id}" styleClass="indexItems">
<f:selectItems value="#{indexBean.myModelValues}" />
<a4j:support event="onchange" reRender="peek,button" />
</h:selectOneMenu>
<br>
<h:outputText id="peek" value ="#{indexBean.model.selected_id}"/>
<br>
<a4j:commandButton id="button" value="Go to Form" action="form" styleClass="indexItems">
<f:param name="selected" value="#{indexBean.model.selected_id}" />
</a4j:commandButton>