具有聚合属性的EJB3 / JPA实体(EJB3/JPA entity with an aggregated attribute)

 我想知道是否有办法在One2Many关系中获得One Side的一个领域，它是Many方面的聚合。 
 
 我们来看下面的例子： 
 
@Entity
public class A {
 @Id
 private Long id;
 @OneToMany (mappedBy="parentA")
 private Collection<B> allBs;

 // Here I don't know how to Map the latest B by date
 private B latestB;
    // Acceptable would be to have : private Date latestBDate;
}

@Entity
public class B {
 @Id
 private Long id;
 private Date date;
 @ManyToOne (targetEntity=A.class)
 private A parentA;
}
 
 我的问题是如何在A实体对象中进行字段latestB的映射而不进行任何去规范化（不与触发器/侦听器保持同步）？ 
 
 也许这个问题给出了一些答案，但实际上我不明白它是如何工作的，因为我仍然希望能够获取所有的子对象。 
 
 感谢阅读/帮助。 
 
 PS：我使用hibernate作为ORM / JPA提供程序，因此如果不存在JPA解决方案，可以提供Hibernate解决方案。 
 PS2：或者只是告诉我，我不应该这样做（当然有论据）;-) 

I wanted to know if there is a way to get in a One2Many relationship a field of the One side that is an aggregate of the Many side.
 
Let's take the following example:
 
@Entity
public class A {
 @Id
 private Long id;
 @OneToMany (mappedBy="parentA")
 private Collection<B> allBs;

 // Here I don't know how to Map the latest B by date
 private B latestB;
    // Acceptable would be to have : private Date latestBDate;
}

@Entity
public class B {
 @Id
 private Long id;
 private Date date;
 @ManyToOne (targetEntity=A.class)
 private A parentA;
}
 
My question is how can I make the mapping of the field latestB in the A entity object without doing any de-normalization (not keeping in sync the field with triggers/listeners)? 
 
Perhaps this question gives some answers, but really I don't understand how it can work since I still want to be able to fetch all childs objects.
 
Thanks for reading/helping.
 
PS: I use hibernate as ORM/JPA provider, so an Hibernate solution can be provided if no JPA solution exists.
 PS2: Or just tell me that I should not do this (with arguments of course) ;-)

原文：https://stackoverflow.com/questions/3709010