如何在hibernate中的多对一映射上定义反级联删除

Question

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.

Answer 1

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.

Answer 2

我认为你需要从A到B的cascade="all,delete-orphan"与one-to-many关联。