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Integrating legacy databases further thing to note is that property-ref requires the target property to be unique, so unique=”true”, as shown earlier, is needed for this mapping. If you try to map this association with JPA annotations, you may look for an equivalent to the property-ref attribute. You map the association with an explicit reference to the natural key column, CUSTOMER_NR: @ManyToOne @JoinColumn(name=”SELLER_NR”, referencedColumnName = “CUSTOMER_NR”) private User seller; Hibernate now knows that the referenced target column is a natural key and manages the foreign key relationship accordingly. To complete this example, you make this association mapping between the two classes bidirectional, with a mapping of an itemsForAuction collection on the User class. First, here it is in XML: …
Again the foreign key column in ITEM is mapped with a property reference to customerNr. In annotations, this is a lot easier to map as an inverse side: @OneToMany(mappedBy = “seller”) private Set itemsForAuction = new HashSet(); Composite foreign key referencing nonprimary keys Some legacy schemas are even more complicated than the one discussed before: A foreign key might be a composite key and, by design, reference a composite natural nonprimary key! Let s assume that USERS has a natural composite key that includes the FIRSTNAME, LASTNAME, and BIRTHDAY columns. A foreign key may reference this natural key, as shown in figure 8.1. To map this, you need to group several properties under the same name otherwise you can t name the composite in a property-ref. Apply the
element to group the mappings:
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