public class Order
{
   public virtual int OrderId {get;set}
   public virtual ISet<Product> Products {get;set}
}

public class Product
{
   public virtual int ProductId {get;set}
   public virtual string ProductName {get;set}
}

DetachedCriteria crit = DetachedCriteria.For<Order>();

crit.Add(Restrictions.Eq("OrderId",orderID);
crit.CreateCriteria("Products","products");
crit.Add(Restrictions.Like("products.ProductName","P%");

crit.List();

.SetResultTransformer(CriteriaUtil.AliasToEntityMap)

<filter name="letterFilter" condition="ProductName like ':letterSupplied'"/>

session.EnableFilter("letterFilter").SetParameter("letterSupplied", "P%");

Order ord = session.CreateCriteria<Order>().Add(Restrictions.IdEq(suppliedId)).UniqueResult<Order>();

<filter name="letterFilter" condition="ProductName like ':letterSupplied%'"/>

<filter name="letterFilter" condition="ProductName like :letterSupplied%"/>

var productIdToSelect = 9;
var crit = Session.CreateCriteria(typeof(Order));
crit.CreateAlias("Product", "prod");
crit.Add(Expression.Eq("prod.Id", productIdToSelect));
var result = crit.List<Order>();