Hibernate 如何使用foregin字段的聚合构建DetachedCriteria？

表结构：

使用SQL聚合商品总量很容易：

SELECT 
    SUM(i.count) as total_goods
FROM operation op
    INNER JOIN order ord 
        ON op.orderId = ord.id
    INNER JOIN item itm 
        ON itm.orderId = ord.id
WHERE
    op.dateCreated < '2015-07-01 00:00:00'

此代码在以下情况下失败：

无法解析属性：order.items.count of:com.Operation

int getTotalGoods(DetachedCriteria filteredOperations) {
   // already contains date filter
   filteredOperations.build {
       projections {
           // 'Order' entity has one-to-many association 'items'
           sum('order.items.count')
       }
   }.list().get(0) ?: 0
}