限制JPA中集合的大小 [英] Limit the size of a collection in JPA

问题描述

说我有一个这样的实体

@Entity
Class A{
//fields

@Onetomany
Set<B> b; // 
}

现在，我如何限制集合中'B'的数量，使得当集合中有新条目时，最旧的条目将被删除，例如

Now, how do I limit the number of 'B's in the collection in such a way that, when there is a new entry in the collection, the oldest one is removed, some thing like removeEldestEntry we have in a LinkedHashMap.

我正在将MySQL 5.5 DB与Hibernate一起使用.预先感谢.

I am using MySQL 5.5 DB with Hibernate. Thanks in advance.

编辑

我的目标是在任何时间点该表中的条目数都不超过N个. 我要解决的一个方法是使用Set并安排作业以删除较旧的条目.但是我发现它很脏.我正在寻找更清洁的解决方案.

My goal is not to have more than N number of entries in that table at any point of time. One solution I have is to use a Set and schedule a job to remove the older entries. But I find it dirty. I am looking for a cleaner solution.

推荐答案

我将使用代码手动执行此规则.主要思想是应该很好地封装集合B，以使客户端只能通过公共方法(即addB())更改其内容.只需在此方法(addB())中确保该规则，即可确保集合B中的条目数不能大于一个值.

I would use the code to manually enforce this rule. The main idea is that the collection B should be well encapsulated such that client only can change its content by a public method (i.e addB()) . Simply ensure this rule inside this method (addB()) to ensure that the number of entries inside the collection B cannot larger than a value.

A:

@Entity
public class A {


    public static int MAX_NUM_B = 4;

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private Set<B> b= new LinkedHashSet<B>();

    public void addB(B b) {
        if (this.b.size() == MAX_NUM_B) {
            Iterator<B> it = this.b.iterator();
            it.next();
            it.remove();
        }
        this.b.add(b);
    }

    public Set<B> getB() {
        return Collections.unmodifiableSet(this.b);
    }
}

B:

@Entity 
public class B{

    @ManyToOne
    private A a;
}

要点:

• A应该是关系的所有者.
• 在A中，不要简单地返回B，因为客户端可以绕过addB(B b)中实现的检查逻辑并自由更改其内容，而是返回B的不可修改视图.
• 在@OneToMany中，将orphanRemoval设置为true，以告诉JPA在从B集合中删除B的相应实例之后，删除B的DB记录.

• A should be the owner of the relationship.
• In A , do not simply return B as client can bypass the checking logic implemented in addB(B b) and change its content freely.Instead , return an unmodifiable view of B .
• In @OneToMany , set orphanRemovalto true to tell JPA to remove the B 's DB records after its corresponding instances are removed from the B collection.

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