selectbox issue

Hi. I'm new in zk. Have a small problem.
In model class i hav a field which has a Set of objects property. I use select box :
< selectbox model="@load(fmm.fClasses)" selectedIndex="@{fmm.fClass.id}" >
< template name="model" var="cl">
${cl.id}
< /template >
< /selectbox >

when a click a command button, selectbox send a selected index, not id value.
how to pass id value via command ? thnx

i found another way to solve my problem, but i still wonder if there's any way to pass not selected index but the id value of my object via selectbox? tnx again