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One problematic inheritance strategy remains: table per concrete class with implicit polymorphism.
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Polymorphic table per concrete class
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In chapter 5, section 5.1.1, Table per concrete class with implicit polymorphism, we defined the table per concrete class mapping strategy and observed that this mapping strategy makes it difficult to represent a polymorphic association, because you can t map a foreign key relationship to a table of the abstract superclass. There is no table for the superclass with this strategy; you have tables only for concrete classes. You also can t create a UNION, because Hibernate doesn t know what unifies the concrete classes; the superclass (or interface) isn t mapped anywhere. Hibernate doesn t support a polymorphic billingDetails one-to-many collection in User, if this inheritance mapping strategy is applied on the BillingDetails hierarchy. If you need polymorphic many-to-one associations with this strategy, you ll have to resort to a hack. The technique we ll show you in this section should be your last choice. Try to switch to a <union-subclass> mapping first. Suppose that you want to represent a polymorphic many-to-one association from User to BillingDetails, where the BillingDetails class hierarchy is mapped with a table per concrete class strategy and implicit polymorphic behavior in Hibernate. You have a CREDIT_CARD table and a BANK_ACCOUNT table, but no BILLING_DETAILS table. Hibernate needs two pieces of information in the USERS table to uniquely identify the associated default CreditCard or BankAccount:
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The name of the table in which the associated instance resides The identifier of the associated instance
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The USERS table requires a DEFAULT_BILLING_DETAILS_TYPE column in addition to the DEFAULT_BILLING_DETAILS_ID. This extra column works as an additional discriminator and requires a Hibernate <any> mapping in User.hbm.xml:
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<any name="defaultBillingDetails" id-type="long" meta-type="string"> <meta-value value="CREDIT_CARD" class="CreditCard"/> <meta-value value="BANK_ACCOUNT" class="BankAccount"/> <column name="DEFAULT_BILLING_DETAILS_TYPE"/> <column name="DEFAULT_BILLING_DETAILS_ID"/> </any>
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The meta-type attribute specifies the Hibernate type of the DEFAULT_BILLING_ DETAILS_TYPE column; the id-type attribute specifies the type of the DEFAULT_
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Advanced entity association mappings
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BILLING_DETAILS_ID column (it s necessary for CreditCard and BankAccount to
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have the same identifier type). The <meta-value> elements tell Hibernate how to interpret the value of the DEFAULT_BILLING_DETAILS_TYPE column. You don t need to use the full table name here you can use any value you like as a type discriminator. For example, you can encode the information in two characters:
<any name="defaultBillingDetails" id-type="long" meta-type="string"> <meta-value value="CC" class="CreditCard"/> <meta-value value="CA" class="BankAccount"/> <column name="DEFAULT_BILLING_DETAILS_TYPE"/> <column name="DEFAULT_BILLING_DETAILS_ID"/> </any>
An example of this table structure is shown in figure 7.15. Here is the first major problem with this kind of association: You can t add a foreign key constraint to the DEFAULT_BILLING_DETAILS_ID column, because some values refer to the BANK_ACCOUNT table and others to the CREDIT_CARD table. Thus, you need to come up with some other way to ensure integrity (a trigger, for example). This is the same issue you d face with a <union-subclass> strategy. Furthermore, it s difficult to write SQL table joins for this association. In particular, the Hibernate query facilities don t support this kind of association mapping, nor may this association be fetched using an outer join. We discourage the use of <any> associations for all but the most special cases. Also note that this mapping
Using a discriminator column with an any association
Summary
technique isn t available with annotations or in Java Persistence (this mapping is so rare that nobody asked for annotation support so far). As you can see, as long as you don t plan to create an association to a class hierarchy mapped with implicit polymorphism, associations are straightforward; you don t usually need to think about it. You may be surprised that we didn t show any JPA or annotation example in the previous sections the runtime behavior is the same, and you don t need any extra mapping to get it.
Summary
In this chapter, you learned how to map more complex entity associations. Many of the techniques we ve shown are rarely needed and may be unnecessary if you can simplify the relationships between your classes. In particular, many-to-many entity associations are often best represented as two one-to-many associations to an intermediate entity class, or with a collection of components. Table 7.1 shows a summary you can use to compare native Hibernate features and Java Persistence.
Table 7.1 Hibernate and JPA comparison chart for chapter 7 Hibernate Core Hibernate supports key generation for shared primary key one-to-one association mappings. Java Persistence and EJB 3.0 Standardized one-to-one mapping is supported. Automatic shared primary key generation is possible through a Hibernate extension. Standardized association mappings are available across secondary tables. Persistent indexes require a Hibernate extension annotation. Fully polymorphic behavior is available, but there is no annotation support for any mappings.
Hibernate supports all entity association mappings across join tables. Hibernate supports mapping of lists with persistent indexes. Hibernate supports fully polymorphic behavior. It provides extra support for any association mappings to an inheritance hierarchy mapped with implicit polymorphism.
In the next chapter, we ll focus on legacy database integration and how you can customize the SQL that Hibernate generates automatically for you. This chapter is interesting not only if you have to work with legacy schemas, but also if you want to improve your new schema with custom DDL, for example.
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