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DELETE FROM ITEMS WHERE item_id = 1
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Just as with the persist and merge methods, the DELETE statement is not necessarily issued immediately but is guaranteed to be issued at some point. Meanwhile, the EntityManager marks the entity as removed so that no further changes to it are synchronized (as we noted in the previous section). Cascading remove operations Just as with merging and persisting entities, you must set the cascade element of a relationship annotation to either ALL or REMOVE for related entities to be removed with the one passed to the remove method. For example, we can specify that the BillingInfo entity related to a Bidder be removed with the owning Bidder entity as follows:
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@Entity public class Bidder { @OneToOne(cascade=CascadeType.REMOVE) public BillingInfo setBillingInfo() {
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From a common usage perspective, this setup makes perfect sense. There is no reason for a BillingInfo entity to hang around if the enclosing Bidder entity it is related to is removed. When it comes down to it, the business domain determines if deletes should be cascaded. In general, you might find that the only relationship types where cascading removal makes sense are one-to-one and one-tomany. You should be careful when using the cascade delete because the related entity you are cascading the delete to may be related to other entities you don t know about. For example, let s assume that there is a one-to-many relationship between the Seller and Item entities and you are using cascade delete to remove a Seller and its related Items. Remember the fact that other entities such as the Category entity also hold references to the Items you are deleting and those relationships would become meaningless!
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Handling relationships If your intent was really to cascade delete the Items associated with the Seller, you should iterate over all instances of Category that reference the deleted Items and remove the relationships first. The following code does this:
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List<Category> categories = getAllCategories(); List<Item> items = seller.getItems(); for (Item item: items) { for (Category category: categories) { category.getItems().remove(item); } } entityManager.remove(seller);
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The code gets all instances of Category in the system and makes sure that all Items related to the Seller being deleted are removed from referencing Lists first. It then proceeds with removing the Seller, cascading the remove to the related Items. Not surprisingly, the remove method must be called from a transactional context or it will throw a TransactionRequiredException. Also, trying to remove an already removed entity will raise IllegalStateException. Having finished the basic EntityManager CRUD operations, let s now move on to the two remaining major persistence operations: flushing data to the database and refreshing from the database.
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We ve been talking about EntityManager flushing on and off throughout the chapter. It is time we discussed this concept fully. For the most part, you ll probably be able to get away without knowing too much about this EntityManager feature. However, there are some situations where not understanding EntityManager flushing could be a great disadvantage. Recall that EntityManager operations like persist, merge, and remove do not cause immediate database changes. Instead, these operations are postponed until the EntityManager is flushed. The true motivation for doing things this way is performance optimization. Batching SQL as much as possible instead of flooding the database with a bunch of frivolous requests saves a lot of communication overhead and avoids unnecessarily tying down the database. By default, the database flush mode is set to AUTO. This means that the EntityManager performs a flush operation automatically as needed. In general, this occurs at the end of a transaction for transaction-scoped EntityManagers and when the persistence context is closed for application-managed or extendedscope EntityManagers. In addition, if entities with pending changes are used in a
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