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What is the effect of cascade on inverse Many new Hibernate users ask this question. The answer is simple: The cascade attribute has nothing to do with the inverse attribute. They often appear on the same collection mapping. If you map a collection of entities as inverse="true", you re controlling the generation of SQL for a bidirectional association mapping. It s a hint that tells Hibernate you mapped the same foreign key column twice. On the other hand, cascading is used as a convenience feature. If you decide to cascade operations from one side of an entity relationship to associated entities, you save the lines of code needed to manage the state of the other side manually. We say that object state becomes transitive. You can cascade state not only on collections of entities, but on all entity association mappings. cascade and inverse have in common the fact that they don t appear on collections of value types or on any other value-type mappings. The rules for these are implied by the nature of value types.
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Are you finished now Well, perhaps not quite. Cascading deletion With the previous mapping, the association between Bid and Item is fairly loose. So far, we have only considered making things persistent as a transitive state. What about deletion It seems reasonable that deletion of an item implies deletion of all bids for the item. In fact, this is what the composition (the filled out diamond) in the UML diagram means. With the current cascading operations, you have to write the following code to make that happen:
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Item anItem = // Load an item // Delete all the referenced bids for ( Iterator<Bid> it = anItem.getBids().iterator(); it.hasNext(); ) { Bid bid = it.next(); it.remove(); session.delete(bid); } session.delete(anItem); // Finally, delete the item // Remove reference from collection // Delete it from the database
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First you remove the references to the bids by iterating the collection. You delete each Bid instance in the database. Finally, the Item is deleted. Iterating and removing the references in the collection seems unnecessary; after all, you ll delete the Item at the end anyway. If you can guarantee that no other object (or
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row in any other table) holds a reference to these bids, you can make the deletion transitive. Hibernate (and JPA) offer a cascading option for this purpose. You can enable cascading for the delete operation:
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<set name="bids" inverse="true" cascade="save-update, delete"> ...
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The operation you cascade in JPA is called remove:
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public class Item { ... @OneToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE, CascadeType.REMOVE }, mappedBy = "item") private Set<Bid> bids = new HashSet<Bid>(); ... }
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The same code to delete an item and all its bids is reduced to the following, in Hibernate or with JPA:
Item anItem = // Load an item session.delete(anItem); entityManager.remove(anItem);
The delete operation is now cascaded to all entities referenced in the collection. You no longer have to worry about removal from the collection and manually deleting those entities one by one. Let s consider one further complication. You may have shared references to the Bid objects. As suggested earlier, a User may have a collection of references to the Bid instances they made. You can t delete an item and all its bids without removing these references first. You may get an exception if you try to commit this transaction, because a foreign key constraint may be violated. You have to chase the pointers. This process can get ugly, as you can see in the following code, which removes all references from all users who have references before deleting the bids and finally the item:
Item anItem = // Load an item // Delete all the referenced bids for ( Iterator<Bid> it = anItem.getBids().iterator(); it.hasNext(); ) {
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