Mapping class inheritance class at runtime) or queries, (Web hosting contract)
Mapping class inheritance class at runtime) or queries, and subclasses declare relatively few properties (particularly if the main difference between subclasses is in their behavior), lean toward table-per-class-hierarchy. Your goal is to minimize the number of nullable columns and to convince yourself (and your DBA) that a denormalized schema won t create problems in the long run. If you do require polymorphic associations or queries, and subclasses declare many properties (subclasses differ mainly by the data they hold), lean toward table-per-subclass. Or, depending on the width and depth of your inheritance hierarchy and the possible cost of joins versus unions, use table-per-concrete-class. By default, choose table-per-class-hierarchy only for simple problems. For more complex cases (or when you re overruled by a data modeler insisting on the importance of nullability constraints and normalization), you should consider the table-per-subclass strategy. But at that point, ask yourself whether it may not be better to remodel inheritance as delegation in the object model. Complex inheritance is often best avoided for all sorts of reasons unrelated to persistence or ORM. Hibernate acts as a buffer between the domain and relational models, but that doesn t mean you can ignore persistence concerns when designing your classes. When you start thinking about mixing inheritance strategies, remember that implicit polymorphism in Hibernate is smart enough to handle more exotic cases. For example, consider an additional interface in our application, Electronic- PaymentOption. This is a business interface that doesn t have a persistence aspect except that in our application, a persistent class such as CreditCard will likely implement this interface. No matter how you map the BillingDetails hierarchy, Hibernate can answer a query from ElectronicPaymentOption correctly. This even works if other classes, which aren t part of the BillingDetails hierarchy, are mapped persistent and implement this interface. Hibernate always know what tables to query, which instances to construct, and how to return a polymorphic result. Finally, you can also use
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