Populate SelectOnemenu filtered by another one (using a4j)

[grdzeli_kaci]

hi all,
i use seam, richfaces and ajax4jsf frameworks for my application,
i have simple question.
i need populate one selectonemenu by another selectonemenu. how i can do it ?
here is my code snippets :
1. View

.............
<h:selectOneMenu id="dbType" styleClass="LoginLangCombo" value="#{dbparams.dbVendor}">
 <s:selectItems value="#{dbVendors.resultList}" var="vendor" label="#{vendor.dbvendorname}" noSelectionLabel="Please Select DataBase ... "/>
 <s:convertEntity />
 <a4j:support event="onchange" reRender="dbDriverid" />
 </h:selectOneMenu>
.............

<h:selectOneMenu id="dbDriverid" styleClass="LoginLangCombo" value="#{dbparams.dbDriver}">
 <s:selectItems value="#{dbDrivers.resultList}" var="driver" label="#{driver.drivername}" noSelectionLabel="Please Select DataBase ... "/>
 <s:convertEntity />
 </h:selectOneMenu>

how i can filter second component by first ?
here is my entities :
2.1 DB_VENDOR

@Entity
@Table(name = "DB_VENDOR",schema="JITS")
public class DbVendor implements Serializable {
private static final long serialVersionUID = 1L;
private BigDecimal id;
private String dbvendorname;
private Collection<DbDriver> dbDrivers;
// .... setters and getters
}

2.2 DB_DRIVER

@Entity
@Table(name = "DB_DRIVER",schema="JITS")
public class DbDriver implements Serializable {
private static final long serialVersionUID = 1L;
private BigDecimal id;
private String dbdrivername;
private DbVendor vendor;
// .... setters and getters
}

and my components.xml looks like :

..........
<framework:entity-home name="dbVendorHome" entity-class="com.magticom.billing.beans.entity.jits.DbVendor"/>
 <factory name="dbVendor" value="#{dbVendorHome.instance}"/>
 <framework:entity-query name="dbVendors" ejbql="select v from DbVendor v" />
..........

i don't know how i can fill second drop down box, filtered by first.

any idea will be appreciated.

______________
Regards
Paata.