如何使用cakephp 3.2内置的求和功能在关联表中进行求和操作(求和字段)，而不是在父表中

我想总结一个字段在cakephp 3.2。但是该字段存在于hasOne模型关联中。

我想在bind查询中做。

我有一个订单表，我正在做模型绑定与收集表。

这里我想把集合表

中所有的due_amount加起来

我试过了，但是没有用。

请检查我的代码，有什么问题吗?

 $this->Orders->hasOne('Collections', [
                'className' => 'Collections',
                'foreignKey' => 'order_id',
                'strategy' => 'select',
                'conditions' => function ('Cake'Database'Expression'QueryExpression $exp, 'Cake'ORM'Query $query) {
                    $query->order(['Collections.id' => 'ASC']);
                    return [];
                }
                    ]);
            $get_total_sales = $this->Orders->find('all')->where($condition)->select(['id', 'region_id', 'net_total'])->contain(['Collections' => ['queryBuilder' => function ($q) {
                                return $q->select(['id', 'order_id', 'total_sale_amount', 'due_amount']);
                            }]])->order(['Orders.due_date DESC']);

$res2 = $get_total_sales->select(['total_due' =>$get_total_sales->func()->sum('collection.due_amount')])->first(); 
 echo  $due = $res2->total_due;//its showing column not find error.
Is it working in order table instead of collections table?
How can i do it for collection table using the $get_total_sales listing results?

Below some out put 
[
    {
        "id": 40,
        "region_id": 2,
        "net_total": 2899.12,
        "collection": {
            "id": 182,
            "order_id": 40,
            "total_sale_amount": 2899.12,
            "due_amount": 1990
        },
    },
    {
        "id": 38,
        "region_id": 2,
        "net_total": 110,
        "collection": {
            "id": 181,
            "order_id": 38,
            "total_sale_amount": 110,
            "due_amount": 10
        },
    },
    {
        "id": 39,
        "region_id": 2,
        "net_total": 16670,
        "collection": {
            "id": 190,
            "order_id": 39,
            "total_sale_amount": 16670,
            "due_amount": 16630.99
        },
    },

这里我想把集合表

中所有的due_amount加起来

Thank you