Single-valued entity associations Let s change the mapping from a User to an Address. Instead of the shared primary key, you now add a SHIPPING_ADDRESS_ID column in the USERS table: The mapping element in XML for this association is not , as you might have expected. The reason is simple: You don t care what s on the target side of the association, so you can treat it like a to-one association without the many part. All you want is to express This entity has a property that is a reference to an instance of another entity and use a foreign key field to represent that relationship. The database schema for this mapping is shown in figure 7.3. Figure 7.3 A one-to-one foreign key association between USERS and ADDRESS An additional constraint enforces this relationship as a real one to one. By making the SHIPPING_ADDRESS_ID column unique, you declare that a particular address can be referenced by at most one user, as a shipping address. This isn t as strong as the guarantee from a shared primary key association, which allows a particular address to be referenced by at most one user, period. With several foreign key columns (let s say you also have unique HOME_ADDRESS_IDand BILLING_ADDRESS_ID), you can reference the same address target row several times. But in any case, two users can t share the same address for the same purpose. Let s make the association from User to Address bidirectional. Inverse property reference The last foreign key association was mapped from Userto Addresswith and a unique constraint to guarantee the desired multiplicity. What mapping
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