使用JPA映射计算的属性 [英] Mapping calculated properties with JPA

问题描述

有没有办法使用JPA映射计算属性？

Is there a way to map a calculated property using JPA?

假设我有一个 Invoice 对象其中有一个或多个 InvoiceLineItems ，我想在 Invoice 类上有一个持久的计算属性，它给了我总数金额：

Assuming I have an Invoice object with one or more InvoiceLineItems within it, I want to have a persistent calculated property on the Invoice class that gives me the total amount:

class Invoice {
    ...

    @Column(name = "TOTAL_AMOUNT")
    public BigDecimal getTotalAmount() {
        BigDecimal amount = BigDecimal.ZERO;
        for (InvoiceLineItem lineItem : lineItems) {
            amount = amount.add(lineItem.getTotalAmount());
        }
        return amount;
    }
}

现在，我可以创建一个受保护的no-op setTotalAmount 的方法，使JPA高兴，但我不知道是否有一种方法可以让JPA知道的映射是唯一的一种方式，以避免产生多余的setter方法。

Now, I could create a protected no-op setTotalAmount method to make JPA happy, but I was wondering if there is a way to let JPA know that the mapping is one way only and to avoid creating a superfluous setter method.

谢谢，
Aleks

Thanks, Aleks

推荐答案

你所描述的不是JPA意义上的计算属性。你在自己的方法中自己计算 - 只需将该方法标记为 @Transient ，JPA将忽略它。

What you've described is not a calculated property in JPA sense. You're calculating it yourself within your method - just mark that method as @Transient and JPA will ignore it.

如果您确实需要计算属性（其中计算表示通过SQL表达式计算），则需要根据JPA提供程序对其进行注释。对于Hibernate，你可以通过 @Formula 注释来实现：

If you truly need a calculated property (where "calculated" means "calculated via SQL expression"), you'll need to annotate it according to your JPA provider. For Hibernate you'd do that via @Formula annotation:

@Formula("col1 * col2")
public int getValue() {
 ...
}

其他提供商可能有自己的配置方式;没有JPA标准。

Other providers may have their own ways to configure this; there's no JPA standard.

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