The Hibernate type system In Hibernate, a value (Web hosting company)
The Hibernate type system In Hibernate, a value type may define associations; it s possible to navigate from a value type instance to some other entity. However, it s never possible to navigate from the other entity back to the value type instance. Associations always point to entities. This means that a value type instance is owned by exactly one entity when it s retrieved from the database; it s never shared. At the level of the database, any table is considered an entity. However, Hibernate provides certain constructs to hide the existence of a database-level entity from the Java code. For example, a many-to-many association mapping hides the intermediate association table from the application. A collection of strings (more accurately, a collection of value-typed instances) behaves like a value type from the point of view of the application; however, it s mapped to its own table. Although these features seem nice at first (they simplify the Java code), we have over time become suspicious of them. Inevitably, these hidden entities end up needing to be exposed to the application as business requirements evolve. The many-to-many association table, for example, often has additional columns added as the application matures. We re almost prepared to recommend that every database- level entity be exposed to the application as an entity class. For example, we would be inclined to model the many-to-many association as two one-to-many associations to an intervening entity class. We ll leave the final decision to you, however, and come back to the topic of many-to-many entity associations in the future chapters. Entity classes are always mapped to the database using
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