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(CreditCard) session.load( CreditCard.class, bd.getId() ); expiryDate = cc.getExpiryDate();
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After the call to load, bd and cc refer to two different proxy instances, which both delegate to the same underlying CreditCard instance. Note that you can avoid these issues by avoiding lazy fetching, as in the following code, using a query technique discussed in the next chapter:
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User user = (User) session.createCriteria(User.class) .add( Expression.eq("id", uid) ) .setFetchMode("billingDetails", FetchMode.EAGER) .uniqueResult(); // The user's billingDetails were fetched eagerly CreditCard cc = (CreditCard) user.getBillingDetails(); expiryDate = cc.getExpiryDate();
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Truly object-oriented code shouldn t use instanceof or numerous typecasts. If you find yourself running into problems with proxies, you should question your design, asking whether there is a more polymorphic approach. One-to-one associations are handled the same way. What about many-valued associations
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6.4.2 Polymorphic collections
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Let s refactor the previous example to its original form in CaveatEmptor. If User owns many BillingDetails, we use a bidirectional one-to-many. In BillingDetails, we have the following:
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<many-to-one name="user" class="User" column="USER_ID"/>
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In the Users mapping, we have this:
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<set name="billingDetails" lazy="true" cascade="save-update" inverse="true"> <key column="USER_ID"/> <one-to-many class="BillingDetails"/> </set>
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Adding a CreditCard is easy:
CreditCard cc = new CreditCard(); cc.setNumber(ccNumber); cc.setType(ccType); cc.setExpiryDate(ccExpiryDate); Session session = sessions.openSession();
Mapping polymorphic associations
Transaction tx = session.beginTransaction(); User user = (User) session.get(User.class, uid); // Call convenience method that sets both "ends" user.addBillingDetails(cc); tx.commit(); session.close();
As usual, addBillingDetails() calls getBillingDetails().add(cc) and cc.setUser(this). We can iterate over the collection and handle instances of CreditCard and BankAccount polymorphically (we don t want to bill users multiple times in our final system, though):
Session session = sessions.openSession(); Transaction tx = session.beginTransaction(); User user = (User) session.get(User.class, uid); Iterator iter = user.getBillingDetails().iterator(); while ( iter.hasNext() ) { BillingDetails bd = (BillingDetails) iter.next(); // Invoke CreditCard.pay() or BankAccount.pay() bd.pay(ccPaymentAmount); } tx.commit(); session.close();
In the examples so far, we ve assumed that BillingDetails is a class mapped explicitly in the Hibernate mapping document, and that the inheritance mapping strategy is table-per-hierarchy or table-per-subclass. We haven t yet considered the case of a table-per-concrete-class mapping strategy, where BillingDetails wouldn t be mentioned explicitly in the mapping file (only in the Java definition of the subclasses).
6.4.3 Polymorphic associations and table-per-concrete-class
In section 3.6.1, Table per concrete class, 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 the table of the abstract superclass. There is no table for the superclass with this strategy; you only have tables for concrete classes. Suppose that we want to represent a polymorphic many-to-one association from User to BillingDetails, where the BillingDetails class hierarchy is mapped using this table-per-concrete-class strategy. There is a CREDIT_CARD table and a
Advanced mapping concepts
BANK_ACCOUNT table, but no BILLING_DETAILS table. We need two pieces of information in the USER table to uniquely identify the associated CreditCard or BankAccount:
The name of the table in which the associated instance resides The identifier of the associated instance
The USER table requires the addition of a BILLING_DETAILS_TYPE column, in addition to the BILLING_DETAILS_ID. We use a Hibernate <any> element to map this association:
<any name="billingDetails" meta-type="string" id-type="long" cascade="save-update"> <meta-value value="CREDIT_CARD" class="CreditCard"/> <meta-value value="BANK_ACCOUNT"class="BankAccount"/> <column name="BILLING_DETAILS_TYPE"/> <column name="BILLING_DETAILS_ID"/> </any>
The meta-type attribute specifies the Hibernate type of the BILLING_DETAILS_TYPE column; the id-type attribute specifies the type of the BILLING_DETAILS_ID column (CreditCard and BankAccount must have the same identifier type). Note that the order of the columns is important: first the type, then the identifier. The <meta-value> elements tell Hibernate how to interpret the value of the BILLING_DETAILS_TYPE column. We don t need to use the full table name here we can use any value we like as a type discriminator. For example, we can encode the information in two characters:
<any name="billingDetails" meta-type="string" id-type="long" cascade="save-update"> <meta-value value="CC" class="CreditCard"/> <meta-value value="CA" class="BankAccount"/> <column name="BILLING_DETAILS_TYPE"/> <column name="BILLING_DETAILS_ID"/> </any>
An example of this table structure is shown in figure 6.14. Here is the first major problem with this kind of association: we can t add a foreign key constraint to the BILLING_DETAILS_ID column, since some values refer to the BANK_ACCOUNT table and others to the CREDIT_CARD table. Thus, we need to come up with some other way to ensure integrity (a trigger, for example).
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