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Manipulating entities with EntityManager
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transaction. If an enclosing transaction is not present when the persist method is invoked, a TransactionRequiredException is thrown for a transaction-scoped entity manager. If you are using an application-managed or extended entity manager, invoking the persist method will attach the entity to persistence context. If a new transaction starts, the entity manager joins the transaction and the changes will be saved to the database when the transaction commits. The same is true for the EntityManager s flush, merge, refresh, and remove methods. Persisting entity relationships One of the most interesting aspects of persistence operations is the handling of entity relationships. JPA gives us a number of options to handle this nuance in a way that suits a particular application-specific situation. Let s explore these options by revisiting listing 9.4. The addItem method is one of the simplest cases of persisting entity relationships. The Seller entity is retrieved using the find method, so it is already managed, and any changes to it are guaranteed to be transparently synchronized. Recall from chapter 7 that there is a bidirectional one-to-many relationship between the Item and Seller entities. This relationship is realized in listing 9.4 by setting the Seller using the item.setSeller method. Let s assume that the Seller entity is mapped to the SELLERS table. Such a relationship between the Item and Seller entities is likely implemented through a foreign key to the SELLERS.SELLER_ID column in the ITEMS table. Since the Seller entity is already persisted, all the EntityManager has to do is set the SELLER_ID foreign key in the generated INSERT statement. Examining the INSERT statement presented earlier, this is how the SELLER_ID value is set to 1. Note that if the seller property of Item were not set at all, the SELLER_ID column in the INSERT statement would be set to NULL. Things become a lot more interesting when we consider the case in which the entity related to the one we are persisting does not yet exist in the database. This does not happen very often for one-to-many and many-to-many relationships. In such cases, the related entity is more than likely already saved in the database. However, it does occur a lot more often for one-to-one relationships. For purposes of illustration, we ll stray from our ItemManager example and take a look at saving User entities with associated BillingInfo entities. Recall that we introduced this unidirectional, one-to-one relationship in chapter 7. The method outlined in listing 9.5 receives user information such as username, e-mail, as well as billing information such as credit card type and number, and persists both the User and related BillingInfo entities into the database.
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Listing 9.5 Persisting relationships
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public User addUser(String username, String email, String creditCardType, String creditCardNumber, Date creditCardExpiration) { User user = new User(); user.setUsername(username); user.setEmail(email); BillingInfo billing = new BillingInfo(); billing.setCreditCardType(creditCardType); billing.setCreditCardNumber(creditCardNumber); billing.setCreditCardExpiration(creditCardExpiration); user.setBillingInfo(billing); entityManager.persist(user); return user; }
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Persists both User and BillingInfo
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As you can see, neither the User entity nor the related BillingInfo entity is managed when the persist method is invoked, since both are newly instantiated. Let s assume for the purpose of this example that the User and BillingInfo entities are saved into the USERS and BILLING_INFO tables, with the one-to-one relationship modeled with a foreign key on the USERS table referencing the BILLING_ID key in the BILLING_INFO table. As you might guess from looking at listing 9.5, two INSERT statements, one for the User and the other for the BillingInfo entity, are issued by JPA. The INSERT statement on the USERS table will contain the appropriate foreign key to the BILLING_INFO table. Cascading persist operations Perhaps surprisingly, it is not the default behavior for JPA to persist related entities. By default, the BillingInfo entity would not be persisted and you would not see an INSERT statement generated to persist the BillingInfo entity into the BILLING_ INFO table. The key to understanding why this is not what happens in listing 9.5 lies in the @OneToOne annotation on the billing property of the User entity:
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public class User { @OneToOne(cascade=CascadeType.PERSIST) public void setBillingInfo(BillingInfo billing) {
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Notice the value of the cascade element of the @OneToOne annotation. We deferred the discussion of this element in chapter 7 so that we could discuss it in a more relevant context here.
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