如何在休眠中的多对一映射上定义反向级联删除 [英] how to define an inverse cascade delete on a many-to-one mapping in hibernate

问题描述

我有两个类 A 和 B.许多 B 可以与单个 A 关联，因此从 B 到 A 是多对一的关系.我已经映射了这种关系:

I have two classes A and B. Many B's can have association with a single A, hence a many-to-one relationship from B to A. I've mapped the relationship like:

<class name="A" table="tbl_A">
  <property name="propA" column="colA"/>
</class>
<class name="B" table="tbl_B">
  <property name="propB" column="colB"/>
  <many-to-one name="a" class="A" column="col1" cascade="delete"/>
</class>

A 没有任何映射到 B.记住这一点，我们打算在 B 关联 A 被删除时删除 B.如果我可以在 B 中的多对一关联上定义一个 inverse="true" 但休眠不允许这样做，这本来是可能的.

A has nothing mapped to B. Keeping this in mind we intend to delete B when it's associated A is deleted. This could have been possible if I could define an inverse="true" on the many-to-one association in B but hibernate does not allow that.

有人可以帮忙吗?我们不想为此在 A 中写任何东西.

Can anyone help with this? We do not want to write anything in A for this.

推荐答案

Hibernate 仅沿定义的关联级联.如果 A 对 B 一无所知，那么你对 A 所做的任何事情都不会影响 B.

Hibernate only cascades along the defined associations. If A knows nothing about Bs, nothing you do with A will affect Bs.

因此，Pascal 的建议是做您想做的事情的最简单方法:

Pascal's suggestion is, therefore, the easiest way to do what you want:

<class name="A" table="tbl_A">
  ...
  <set name="myBs" inverse="true" cascade="all,delete-orphan">
    <key column="col1"/>
    <one-to-many class="B"/>
  </set>
</class>

<class name="B" table="tbl_B">
  ...
  <many-to-one name="a" class="A" column="col1" not-null="true"/>
</class>

请注意，在 B 上设置 cascade="delete" 就像在原始代码中一样不会执行您想要的操作 - 它告诉 Hibernate如果B 被删除"，这可能会导致违反约束(如果有任何其他 B 链接到该 A).

Note that setting cascade="delete" on B as you have it in your original code will NOT do what you want - it tells Hibernate to "delete A if B is deleted" which is likely to result in constraint violation (if there are any other Bs linked to that A).

如果您绝对不能将 B 的集合添加到 A(尽管我无法真正想到会出现这种情况的情况)，那么您唯一的另一种选择是在外部定义从 A 到 B 的级联删除关键级别；当你的 A 被删除时，你的 B 也会被删除.

If you absolutely cannot add a collection of Bs to A (though I can't really think of the circumstances where that'd be the case), your only other alternative is to define cascade delete from A to B at the foreign key level; your Bs will then be deleted when your A is deleted.

然而，这是一个相当丑陋的解决方案，因为您必须非常小心在 Hibernate 中删除 A 的方式:

This is a rather ugly solution, however, because you have to be extremely careful of how you delete A in Hibernate:

1. 在删除 A 之前必须刷新会话(对 B 进行挂起的更新可能会导致错误或 A 和一些 B 在幕后重新插入)
2. 必须从所有活动会话和二级缓存中逐出所有链接到您的 A 的 B(并且由于您没有维护 A 端的关系，这意味着所有 B)必须被逐出.

1. Session must be flushed prior to deleting A (having pending updates to B may result in an error or A and some Bs being re-inserted behind the scenes)
2. All Bs linked to your A (and since you're not maintaining the relationship from A side that means all Bs) must be evicted from all active sessions and 2nd level cache.

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