原则2：如果没有选择至少一个根实体别名，则无法通过标识变量选择实体

问题描述

这不是我的实际问题，而是对我自己的便笺，可能会对他人有所帮助。还有许多其他类似的问题：，，，，，，但似乎没有一个提供此解决方案。

Rather than an actual question, this is a sticky note for myself, that may help others. There are many other similar questions: 1, 2, 3, 4, 5, 6, but none of them seems to offer this solution.

我有以下实体：

class Order
{
    // ...

    /**
     * @ManyToOne(targetEntity="Customer")
     * @var Customer
     */
    private $customer;

    /**
     * @Column(type="integer")
     * @var int
     */
    private $amount;
}

class Customer
{
    // ...
}

Order 与 Customer 。我想获得每个客户及其订单总额，所以我运行以下DQL查询：

Order has a unidirectional, many-to-one relationship with Customer. I want to get every customer, along with the total amount of his orders, so I run the following DQL query:

SELECT c, SUM(o.amount)
FROM Model\Order o
JOIN o.customer c
GROUP BY c

但出现以下错误：

我该如何解决？

推荐答案

这是一个已知的原则限制。

This is a known Doctrine limitation.

解决方案是显式地 SELECT 您想要的实体检索（ Customer ）并使用从那里手动加入另一个实体（ Order ）有条件：

The solution is to explicitly SELECT the entity you want to retrieve (Customer) and manually join the other entity (Order) from there, using a WITH condition:

SELECT c, SUM(o.amount)
FROM Model\Customer c
JOIN Model\Order o WITH o.customer = c
GROUP BY c

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