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guaranteed object identity; and we ll come back to this subject in chapter 9, section 9.2, Object identity and equality. Using database identifiers in Hibernate is easy and straightforward. Choosing a good primary key (and key-generation strategy) may be more difficult. We discuss this issue next.
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Hibernate needs to know your preferred strategy for generating primary keys. First, though, let s define primary key. Selecting a primary key The candidate key is a column or set of columns that could be used to identify a particular row in a table. To become a primary key, a candidate key must satisfy the following properties:
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Its value (for any column of the candidate key) is never null. Each row has a unique value. The value of a particular row never changes.
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If a table has only one identifying attribute, it s, by definition, the primary key. However, several columns or combinations of columns may satisfy these properties for a particular table; you choose between candidate keys to decide the best primary key for the table. Candidate keys not chosen as the primary key should be declared as unique keys in the database. Many legacy SQL data models use natural primary keys. A natural key is a key with business meaning: an attribute or combination of attributes that is unique by virtue of its business semantics. Examples of natural keys are the U.S. Social Security Number and Australian Tax File Number. Distinguishing natural keys is simple: If a candidate key attribute has meaning outside the database context, it s a natural key, whether or not it s automatically generated. Think about the application users: If they refer to a key attribute when talking about and working with the application, it s a natural key. Experience has shown that natural keys almost always cause problems in the long run. A good primary key must be unique, constant, and required (never null or unknown). Few entity attributes satisfy these requirements, and some that do can t be efficiently indexed by SQL databases (although this is an implementation detail and shouldn t be the primary motivation for or against a particular key). In
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addition, you should make certain that a candidate key definition can never change throughout the lifetime of the database before making it a primary key. Changing the value (or even definition) of a primary key, and all foreign keys that refer to it, is a frustrating task. Furthermore, natural candidate keys can often be found only by combining several columns in a composite natural key. These composite keys, although certainly appropriate for some relations (like a link table in a many-to-many relationship), usually make maintenance, ad-hoc queries, and schema evolution much more difficult. For these reasons, we strongly recommend that you consider synthetic identifiers, also called surrogate keys. Surrogate keys have no business meaning they re unique values generated by the database or application. Application users ideally don t see or refer to these key values; they re part of the system internals. Introducing a surrogate key column is also appropriate in a common situation: If there are no candidate keys, a table is by definition not a relation as defined by the relational model it permits duplicate rows and so you have to add a surrogate key column. There are a number of well-known approaches to generating surrogate key values. Selecting a key generator Hibernate has several built-in identifier-generation strategies. We list the most useful options in table 4.1.
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Table 4.1 Hibernate s built-in identifier-generator modules JPA GenerationType Options Description The native identity generator picks other identity generators like identity, sequence, or hilo, depending on the capabilities of the underlying database. Use this generator to keep your mapping metadata portable to different database management systems. This generator supports identity columns in DB2, MySQL, MS SQL Server, Sybase, and HypersonicSQL. The returned identifier is of type long, short, or int.
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