data table multiple check box does not keep selection - primefaces

Question

I'm using datatable with multiple selection and pagination, when I select a row and go to another page and then return to the page I was the row I've selected is not selected anymore. I'm using primefaces 3.5, mojarra, jboss 7.1, my bean is viewScoped. Below is my code:
<p:dataTable id="boxList" var="box" value="#{protocolBean.boxModel}" paginator="true" rows="10" paginatorPosition="bottom"
selection="#{protocolBean.selectedBoxes}">
<f:facet name="header">
#{label['boxes']}
</f:facet>
<p:column selectionMode="multiple" style="width:4%" />
<p:column>
<h:outputText value="#{box.code}"/>
</p:column>
<p:column filterBy="#{box.selected}" filterOptions="#{protocolBean.selectedOptions}" filterMatchMode="exact">
<h:outputText value="#{box.selected}"/>
</p:column>
</p:dataTable>
Model:
public class BoxModel extends ListDataModel<Box> implements SelectableDataModel<Box> {
public BoxModel() {
}
public BoxModel(List<Box> boxes) {
super(boxes);
}
#Override
public Object getRowKey(Box box) {
return box.getId();
}
#SuppressWarnings("unchecked")
#Override
public Box getRowData(String rowKey) {
List<Box> boxes = (List<Box>) getWrappedData();
for(Box b : boxes) {
if(b.getId().equals(rowKey))
return b;
}
return null;
}
}

Answer 1

I discovered what was happening, the problem is with my model class, in method getRowData I'm comparing a long (b.getId) with a String (rowKey) this way the method always return null and will never know who is selected.