Spring form model attribute class with another object - forms

Question

I work with Spring MVC and JSP. I want to input the class product using a form. Class Product has another object Catalog
Product class
public class Product {
private int productId;
private Catalog catalog;
private String sku;
private String productName;
private int availableQuantity;
private String uom;
public Product() {
}
Catalog is
public class Catalog {
protected static Logger logger = Logger.getLogger(Catalog.class.getName());
private int catalogId;
private String catalogName;
private List<Product> products= new ArrayList<Product>();
public Catalog() {
}
I created a modelAttribute product in the controller
model.addAttribute(new Product());
return "product/addProduct";
I want to input the catalog id using a dropdown. This dropdown is
<tr>
<th><label for="catalog_id">Catalog id:</label></th>
<td><sf:select path="catalog" items="${catalogList}" />
</td>
</tr>
The jsp is:
<sf:form method="POST" modelAttribute="product">
<table border="0">
<tr>
<th><label for="catalog_id">Catalog id:</label></th>
<td><sf:select path="catalog" items="${catalogList}" />
</td>
</tr>
<tr>
<th><label for="product_name">Product Name:</label></th>
<td><sf:input path="productName" size="128" id="product_name" /><br/>
<sf:errors path="productName" cssClass="error" />
</td>
</tr>
<tr>
<th><label for="product_sku">Product sku:</label></th>
<td><sf:input path="sku" size="32" id="product_sku" /><br/>
<sf:errors path="sku" cssClass="error" />
</td>
</tr>
<tr>
<th><label for="product_availablequantity">Available Quantity:</label></th>
<td><sf:input path="availableQuantity" id="product_availablequantity" /><br/>
<sf:errors path="availableQuantity" cssClass="error" />
</td>
</tr>
<tr>
<th><label for="product_unitofmeasure">Unit of measure:</label></th>
<td><sf:input path="uom" size="32" id="product_unitofmeasure" /><br/>
<sf:errors path="uom" cssClass="error" />
</td>
</tr>
<tr>
<th></th>
<td><input type="submit" value="Save" /></td>
</tr>
</table>
</sf:form>
</body>
</html>
How do I have to define the path product.catalog.catalogId?