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Mapping collections and entity associations
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Bid bid = it.next(); // Remove references from users who have made this bid Query q = session.createQuery( "from User u where :bid in elements(u.bids)" ); q.setParameter("bid", bid); Collection usersWithThisBid = q.list(); for (Iterator itUsers = usersWithThisBid.iterator(); itUsers.hasNext();) { User user = (User) itUsers.next(); user.getBids().remove(bid); } } session.delete(anItem); // Finally, delete the item and the associated bids
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Obviously, the additional query (in fact, many queries) isn t what you want. However, in a network object model, you don t have any choice other than executing code like this if you want to correctly set pointers and references there is no persistent garbage collector or other automatic mechanism. No Hibernate cascading option helps you; you have to chase all references to an entity before you finally delete it. (This isn t the whole truth: Because the BIDDER_ID foreign key column that represents the association from User to Bid is in the BID table, these references are automatically removed at the database level if a row in the BID table is deleted. This doesn t affect any objects that are already present in memory in the current unit of work, and it also doesn t work if BIDDER_ID is mapped to a different (intermediate) table. To make sure all references and foreign key columns are nulled out, you need to chase pointers in Java.) On the other hand, if you don t have shared references to an entity, you should rethink your mapping and map the bids as a collection components (with the Bid as a <composite-element>). With an <idbag> mapping, even the tables look the same:
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<class name="Item" table="ITEM"> ... <idbag name="bids" table="BID"> <collection-id type="long" column="BID_ID"> <generator class="sequence"/> </collection-id>
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Mapping a parent/children relationship
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<key column="ITEM_ID" not-null="true"/> <composite-element class="Bid"> <parent name="item"/> <property .../> ... </composite-element> </idbag> </class>
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The separate mapping for Bid is no longer needed. If you really want to make this a one-to-many entity association, Hibernate offers another convenience option you may be interested in. Enabling orphan deletion The cascading option we explain now is somewhat difficult to understand. If you followed the discussion in the previous section, you should be prepared. Imagine you want to delete a Bid from the database. Note that you aren t deleting the parent (the Item) in this case. The goal is to remove a row in the BID table. Look at this code:
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anItem.getBids().remove(aBid);
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If the collection has the Bid mapped as a collection of components, as in the previous section, this code triggers several operations:
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The aBid instance is removed from the collection Item.bids. Because Bid is mapped as a value type, and no other object can hold a reference to the aBid instance, the row representing this bid is deleted from the BID table by Hibernate.
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In other words, Hibernate assumes that aBid is an orphan if it s removed from its owning entity s collection. No other in-memory persistent object is holding a reference to it. No foreign key value that references this row can be present in the database. Obviously, you designed your object model and mapping this way by making the Bid class an embeddable component. However, what if Bid is mapped as an entity and the collection is a <one-tomany> The code changes to
anItem.getBids().remove(aBid); session.delete(aBid);
The aBid instance has its own lifecycle, so it can exist outside of the collection. By deleting it manually, you guarantee that nobody else will hold a reference to it,
Mapping collections and entity associations
and the row can be removed safely. You may have removed all other references manually. Or, if you didn t, the database constraints prevent any inconsistency, and you see a foreign key constraint exception. Hibernate offers you a way to declare this guarantee for collections of entity references. You can tell Hibernate, If I remove an element from this collection, it will be an entity reference, and it s going to be the only reference to that entity instance. You can safely delete it. The code that worked for deletion with a collection of components works with collections of entity references. This option is called cascade orphan delete. You can enable it on a collection mapping in XML as follows:
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