Cascade 和 Inverse

Set 中 cascade属性：

cascade=" all|none|save-update|delete|all-delete-orphan|delete-orphan"

none：不作任何级联操作
save-update： 显然级联save，update，saveOrUpdate
delete： 显然级联删除
all-delete-orphan： 测试过，包括级联save，update，savaOrUpdate，delete。而且更新主表的集合，自动把子表记录删掉，而不是设置外键列为null
其他：  不知道啊


下面摘自Hibernate 文档：

21.3. Cascading lifecycle

The explicit call to save() is still annoying. We will address this by using cascades.

<set name="children" inverse="true" cascade="all">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

This simplifies the code above to

Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();

Similarly, we don't need to iterate over the children when saving or deleting a Parent. The following removes p and all its children from the database.

Parent p = (Parent) session.load(Parent.class, pid);
session.delete(p);
session.flush();

However, this code

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();

will not remove c from the database; it will ony remove the link to p (and cause a NOT NULL constraint violation, in this case). You need to explicitly delete() the Child.

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();

Now, in our case, a Child can't really exist without its parent. So if we remove a Child from the collection, we really do want it to be deleted. For this, we must use cascade="all-delete-orphan".

<set name="children" inverse="true" cascade="">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

Note: even though the collection mapping specifies inverse="true", cascades are still processed by iterating the collection elements. So if you require that an object be saved, deleted or updated by cascade, you must add it to the collection. It is not enough to simply call setParent().

21.4. Cascades and unsaved-value

Suppose we loaded up a Parent in one Session, made some changes in a UI action and wish to persist these changes in a new session by calling update(). The Parent will contain a collection of childen and, since cascading update is enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in the database. Lets assume that both Parent and Child have genenerated identifier properties of type Long. Hibernate will use the identifier and version/timestamp property value to determine which of the children are new. (See Section 10.7, “Automatic state detection”.) In Hibernate3, it is no longer necessary to specify an unsaved-value explicitly.

The following code will update parent and child and insert newChild.

//parent and child were both loaded in a previous session
parent.addChild(child);
Child newChild = new Child();
parent.addChild(newChild);
session.update(parent);
session.flush();

Well, that's all very well for the case of a generated identifier, but what about assigned identifiers and composite identifiers? This is more difficult, since Hibernate can't use the identifier property to distinguish between a newly instantiated object (with an identifier assigned by the user) and an object loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if the row exists.

21.5. Conclusion

There is quite a bit to digest here and it might look confusing first time around. However, in practice, it all works out very nicely. Most Hibernate applications use the parent / child pattern in many places.

We mentioned an alternative in the first paragraph. None of the above issues exist in the case of <composite-element> mappings, which have exactly the semantics of a parent / child relationship. Unfortunately, there are two big limitations to composite element classes: composite elements may not own collections, and they should not be the child of any entity other than the unique parent.