CHAPTER 7 Advanced entity association mappings element can you add on the Addressside to make this association bidirectional, so that access from Address to User is possible in the Java domain model? In XML, you create a mapping with a property reference attribute: You tell Hibernate that the user property of the Address class is the inverse of a property on the other side of the association. You can now call anAddress.getUser() to access the user who s shipping address you ve given. There is no additional column or foreign key constraint; Hibernate manages this pointer for you. Should you make this association bidirectional? As always, the decision is up to you and depends on whether you need to navigate through your objects in that direction in your application code. In this case, we d probably conclude that the bidirectional association doesn t make much sense. If you call anAddress.getUser(), you are saying give me the user who has this address has its shipping address, not a very reasonable request. We recommend that a foreign key-based one-to-one association, with a unique constraint on the foreign key column is almost always best represented without a mapping on the other side. Let s repeat the same mapping with annotations. Mapping a foreign key with annotations The JPA mapping annotations also support a one-to-one relationship between entities based on a foreign key column. The main difference compared to the mappings earlier in this chapter is the use of @JoinColumn instead of @PrimaryKeyJoinColumn. First, here s the to-one mapping from User to Address with the unique constraint on the SHIPPING_ADDRESS_ID foreign key column. However, instead of a @ManyToOne annotation, this requires a @OneToOne annotation: public class User { … @OneToOne @JoinColumn(name=”SHIPPING_ADDRESS_ID”) private Address shippingAddress; … }
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