21.3. Cascading life cycle

You can address the frustrations of the explicit call to save() 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 do not 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, the following 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. In this case, it will only remove the link to p and cause a NOT NULL constraint violation. 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();
In our case, a Child cannot exist without its parent. So if we remove a Child from the collection, we do want it to be deleted. To do this, we must use cascade="all-delete-orphan".
<set name="children" inverse="true" cascade="all-delete-orphan">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>
Even though the collection mapping specifies inverse="true", cascades are still processed by iterating the collection elements. If you need an object be saved, deleted or updated by cascade, you must add it to the collection. It is not enough to simply call setParent().