CHAPTER 1 Understanding object/relational persistence its parent. However, (Sex offenders web site)
CHAPTER 1 Understanding object/relational persistence its parent. However, such a feature would be questionable, because it would introduce a new notion: virtual columns in base tables. Traditionally, we expect virtual columns only in virtual tables, which are called views. Furthermore, on a theoretical level, the inheritance we applied in Java is type inheritance. A table isn t a type, so the notion of supertables and subtables is questionable. In any case, we can take the short route here and observe that SQL database products don t generally implement type or table inheritance, and if they do implement it, they don t follow a standard syntax and usually expose you to data integrity problems (limited integrity rules for updatable views). In chapter 5, section 5.1, Mapping class inheritance, we discuss how ORM solutions such as Hibernate solve the problem of persisting a class hierarchy to a database table or tables. This problem is now well understood in the community, and most solutions support approximately the same functionality. But we aren t finished with inheritance. As soon as we introduce inheritance into the model, we have the possibility of polymorphism. The User class has an association to the BillingDetails superclass. This is a polymorphic association. At runtime, a User object may reference an instance of any of the subclasses of BillingDetails. Similarly, we want to be able to write polymorphic queries that refer to the BillingDetails class, and have the query return instances of its subclasses. SQL databases also lack an obvious way (or at least a standardized way) to represent a polymorphic association. A foreign key constraint refers to exactly one target table; it isn t straightforward to define a foreign key that refers to multiple tables. We d have to write a procedural constraint to enforce this kind of integrity rule. The result of this mismatch of subtypes is that the inheritance structure in your model must be persisted in an SQL database that doesn t offer an inheritance strategy. Fortunately, three of the inheritance mapping solutions we show in chapter 5 are designed to accommodate the representation of polymorphic associations and the efficient execution of polymorphic queries. The next aspect of the object/relational mismatch problem is the issue of object identity. You probably noticed that we defined USERNAME as the primary key of our USERS table. Was that a good choice? How do we handle identical objects in Java? 1.2.3 The problem of identity Although the problem of object identity may not be obvious at first, we ll encounter it often in our growing and expanding e-commerce system, such as when we need to check whether two objects are identical. There are three ways to tackle
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