Java 如何在休眠中的多对一映射上定义反向级联删除
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how to define an inverse cascade delete on a many-to-one mapping in hibernate
提问by Monis Iqbal
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:
我有两个类 A 和 B。许多 B 可以与单个 A 相关联，因此从 B 到 A 是多对一的关系。我已经将这种关系映射为：
<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 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.
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" ，这本来是可能的，但休眠不允许这样做。
Can anyone help with this? We do not want to write anything in A for this.
有人能帮忙吗？我们不想为此在 A 中写任何东西。
采纳答案by ChssPly76
Hibernate only cascades along the defined associations. If A knows nothing about Bs, nothing you do with A will affect Bs.
Hibernate 仅沿定义的关联级联。如果 A 对 B 一无所知，那么您对 A 所做的任何事情都不会影响 B。
Pascal's suggestion is, therefore, the easiest way to do what you want:
因此，Pascal 的建议是做你想做的最简单的方法：
<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>
Note that setting cascade="delete"on Bas 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).
请注意，设置cascade="delete"上B，你必须在你原来的代码不会做你想要的东西-它告诉Hibernate“删除，如果B被删除，”这很可能导致违反约束（如果有链接到一个任何其他烧烤）。
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.
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键级别定义从 A 到 B 的级联删除；当您的 A 被删除时，您的 B 将被删除。
This is a rather ugly solution, however, because you have to be extremely careful of how you delete A in Hibernate:
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：
- 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)
- All Bs linked to your A (and since you're not maintaining the relationship from A side that means allBs) must be evicted from all active sessions and 2nd level cache.
- 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）
- 链接到您的 A 的所有 B（并且由于您没有维护 A 端的关系，这意味着所有B）必须从所有活动会话和二级缓存中驱逐。
回答by Pascal Thivent
I think you need to cascade="all,delete-orphan"from A to B's with a one-to-manyassociation.
我认为您需要cascade="all,delete-orphan"通过one-to-many关联从 A 到 B。
