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CHAPTER 8 Legacy databases and custom SQL more often relates to the implementation of the logical data model as a physical database schema. If you accept this observation, you ll see that the kinds of problems that require schema changes are those that necessitate addition of new entities, refactoring of existing entities, addition of new attributes to existing entities, and modification to the associations between entities. The problems that can be solved without schema changes usually involve inconvenient table or column definitions for a particular entity. In this section, we ll concentrate on these kinds of problems. We assume that you ve tried to reverse-engineer your existing schema with the Hibernate toolset, as described in chapter 2, section 2.3, Reverse engineering a legacy database. The concepts and solutions discussed in the following sections assume that you have basic object/relational mapping in place and that you need to make additional changes to get it working. Alternatively, you can try to write the mapping completely by hand without the reverse-engineering tools. Let s start with the most obvious problem: legacy primary keys. 8.1.1 Handling primary keys We ve already mentioned that we think natural primary keys can be a bad idea. Natural keys often make it difficult to refactor the data model when business requirements change. They may even, in extreme cases, impact performance. Unfortunately, many legacy schemas use (natural) composite keys heavily and, for the reason we discourage the use of composite keys, it may be difficult to change the legacy schema to use noncomposite natural or surrogate keys. Therefore, Hibernate supports the use of natural keys. If the natural key is a composite key, support is via the mapping. Let s map both a composite and a noncomposite natural primary key. Mapping a natural key If you encountered a USERS table in a legacy schema, it s likely that USERNAME is the actual primary key. In this case, you have no surrogate identifier that is automatically generated. Instead, you enable the assigned identifier generator strategy to indicate to Hibernate that the identifier is a natural key assigned by the application before the object is saved: …
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