What are the inheritance mapping models in Hibernate?

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Hi we want to know that What are the inheritance mapping models in Hibernate?

Satyapriya Nayak · Answer

Hi Abhishek Inheritance mapping The three strategies Hibernate supports the three basic inheritance mapping strategies: table per class hierarchy table per subclass table per concrete class In addition Hibernate supports a fourth slightly different kind of polymorphism: implicit polymorphism It is possible to use different mapping strategies for different branches of the same inheritance hierarchy. You can then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However Hibernate does not support mixing  and mappings under the same root element. It is possible to mix together the table per hierarchy and table per subclass strategies under the the same element by combining the and elements (see below for an example). It is possible to define subclass  union-subclass  and joined-subclass mappings in separate mapping documents directly beneath hibernate-mapping . This allows you to extend a class hierarchy by adding a new mapping file. You must specify an extends attribute in the subclass mapping naming a previously mapped superclass. Previously this feature made the ordering of the mapping documents important. Since Hibernate3 the ordering of mapping files is irrelevant when using the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses. Table per class hierarchy Suppose we have an interface Payment with the implementors CreditCardPayment  CashPayment  and ChequePayment . The table per hierarchy mapping would display in the following way: ... ... ... ... Exactly one table is required. There is a limitation of this mapping strategy: columns declared by the subclasses such as CCTYPE  cannot have NOT NULL constraints. Table per subclass A table per subclass mapping looks like this: ... ... ... ... Four tables are required. The three subclass tables have primary key associations to the superclass table so the relational model is actually a one-to-one association. Table per subclass: using a discriminator Hibernate's implementation of table per subclass does not require a discriminator column. Other object/relational mappers use a different implementation of table per subclass that requires a type discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to implement but arguably more correct from a relational point of view. If you want to use a discriminator column with the table per subclass strategy you can combine the use of and  as follows: ... ... ... ... The optional fetch=select declaration tells Hibernate not to fetch the ChequePayment subclass data using an outer join when querying the superclass. Mixing table per class hierarchy with table per subclass You can even mix the table per hierarchy and table per subclass strategies using the following approach: ... ... ... ... For any of these mapping strategies a polymorphic association to the root Payment class is mapped using . Table per concrete class There are two ways we can map the table per concrete class strategy. First you can use . ... ... ... ... Three tables are involved for the subclasses. Each table defines columns for all properties of the class including inherited properties. The limitation of this approach is that if a property is mapped on the superclass the column name must be the same on all subclass tables. The identity generator strategy is not allowed in union subclass inheritance. The primary key seed has to be shared across all unioned subclasses of a hierarchy. If your superclass is abstract map it with abstract=true . If it is not abstract an additional table (it defaults to PAYMENT in the example above) is needed to hold instances of the superclass. Table per concrete class using implicit polymorphism An alternative approach is to make use of implicit polymorphism: ... ... ... Notice that the Payment interface is not mentioned explicitly. Also notice that properties of Payment are mapped in each of the subclasses. If you want to avoid duplication consider using XML entities (for example [ ] in the DOCTYPE declaration and &allproperties; in the mapping). The disadvantage of this approach is that Hibernate does not generate SQL UNION s when performing polymorphic queries. For this mapping strategy a polymorphic association to Payment is usually mapped using . Mixing implicit polymorphism with other inheritance mappings Since the subclasses are each mapped in their own element and since Payment is just an interface) each of the subclasses could easily be part of another inheritance hierarchy. You can still use polymorphic queries against the Payment interface. ... ... ... ... Once again Payment is not mentioned explicitly. If we execute a query against the Payment interface for example from Payment  Hibernate automatically returns instances of CreditCardPayment (and its subclasses since they also implement Payment ) CashPayment and ChequePayment  but not instances of NonelectronicTransaction . Limitations There are limitations to the implicit polymorphism approach to the table per concrete-class mapping strategy. There are somewhat less restrictive limitations to mappings. The following table shows the limitations of table per concrete-class mappings and of implicit polymorphism in Hibernate. Table 9.1. Features of inheritance mappings Inheritance strategy Polymorphic many-to-one Polymorphic one-to-one Polymorphic one-to-many Polymorphic many-to-many Polymorphic load()/get() Polymorphic queries Polymorphic joins Outer join fetching table per class-hierarchy s.get(Payment.class id) from Payment p from Order o join o.payment p supported table per subclass s.get(Payment.class id) from Payment p from Order o join o.payment p supported table per concrete-class (union-subclass) (for inverse=true only) s.get(Payment.class id) from Payment p from Order o join o.payment p supported table per concrete class (implicit polymorphism) not supported not supported s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult() from Payment p not supported not supported Please refer the below link http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/inheritance.html http://www.ibm.com/developerworks/java/library/j-hibernate/ Thanks