如何在休眠中的多对一映射中定义逆级联删除 [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有关联，因此是从B到A的多对一关系。我已经将关系映射为：

 < class name =Atable =tbl_A> 
< property name =propAcolumn =colA/> 
< / class> 
< class name =Btable =tbl_B> 
< property name =propBcolumn =colB/> 
 < / class> 
  

A没有任何映射B.保持这一点，我们打算删除B时，它是关联的是删除。如果我可以在B中的多对一关联中定义一个inverse =true，但是hibernate不允许的话，这可能是可能的。

任何人都可以帮助有了这个？我们不希望为此写入任何内容。

解决方案

Hibernate只能沿着定义的关联进行级联。如果A对Bs一无所知，那么你对A所做的任何事情都不会影响Bs。

因此，帕斯卡的建议是最简单的方法来做你想做的事情：

 < class name =Atable =tbl_A> 
 ... 
< set name =myBsinverse =truecascade =all，delete-orphan> 
< key column =col1/> 
<一对多课程=B/> 
< / set> 
< / class> 
 
< class name =Btable =tbl_B> 
 ... 
 < / class> 
  

请注意，设置 cascade =delete在 B 上，因为你在原始代码中拥有它将不会做你想做的事情 - 它告诉Hibernate删除A，如果B被删除，这很可能导致违反约束（如果有任何其他Bs与该A链接）。

如果绝对不能将A的集合添加到A（尽管我无法真正想到这种情况在那种情况下），唯一的另一种选择是在外键级别定义级联从A删除到B;当你的A被删除时，你的Bs将被删除。

然而，这是一个相当难看的解决方案，因为你必须非常小心你如何在Hibernate中删除A ：

1. 在删除A之前，会话必须刷新（具有挂起的更新到B可能会导致错误或A，插入幕后）


2. 必须将所有与您的A链接的B（并且因为您没有保持与A方的关系，这意味着所有 B）必须被驱逐来自所有活动会话和二级缓存。

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 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.

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

解决方案

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

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>

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).

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.

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

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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