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What s new in Hibernate 3
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<discriminator type="string"> <formula> case when class_type = 'network' then 'NetworkEvent' else 'Event' end </formula> </discriminator> <discriminator type="string" formula=" "/>
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This example returns a string indicating the class type using a CASE statement. The returned string is actually the name of the class. Listing 11.1 shows a more complete mapping definition using derived discriminator values. Listing 11.1 Derived discriminators
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<hibernate-mapping> <class name="Event" discriminator-value="event"> <discriminator type="string"> <formula> case class_type when 'event' then 'Event' when 'network' then 'NetworkEvent' when 'food' then 'FoodEvent' end </formula> </discriminator> <subclass name="NetworkEvent" discriminator-value="network"> </subclass> <subclass name="FoodEvent" discriminator-value="food"> </subclass> </class> </hibernate-mapping>
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Mapping improvements
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We still have a discriminator value, but it is derived from other values rather than being concrete. Derived discriminator values come into play with our next topic, union subclasses.
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11.2.3 Union subclasses
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When we discussed the various inheritance mapping strategies in chapter 3, one strategy we didn t discuss was the table-per-concreteclass concept. We avoided it because it is complicated to implement in Hibernate 2, and it does not support a number of polymorphic features, such as polymorphic one-to-many and joins, as well as outerjoin fetching.
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subclass>
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To address this shortcoming, Hibernate 3 introduces the new <unionelement. Classes mapped with this element use the tableper-concrete-class strategy, but don t have the same limitations on polymorphic operations and joins. The mapping for a union-subclass is shown here:
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<hibernate-mapping> <class name="Event"> <union-subclass name="NetworkEvent"> </union-subclass> <union-subclass name="FoodEvent"> </union-subclass> </class> </hibernate-mapping>
Like the former table-per-concrete-class strategy, each subclass table contains all of the property columns, including the inherited fields. The primary difference is that the former method mapped all classes with the class element, which didn t preserve the inheritance hierarchy. The <union-subclass> element makes the table-per-concrete-class strategy much more usable and powerful.
What s new in Hibernate 3
11.2.4 Property references
When you re creating an association between two objects, Hibernate assumes that the association will be from the primary key of one object to the foreign key of the associated object. However, this isn t always the case, particularly in legacy database schema. To address non-primary key associations, Hibernate 3 adds the property-ref attribute to the association elements, including <many-to-one> and <one-to-one>. This feature is best explained with an example. Suppose you want to associate your Event class to the Location using the name of the Location, rather than the id. In that case, the <many-toone> element in the Event definition would be
<many-to-one class="Location" property-ref="name"/>
The requirement on the referenced property is that it must be unique. Otherwise, the Event class could be associated to multiple Locations. Because of this limitation, the mapping definition for the Location class has to be modified:
<property name="name" unique="true"/>
If your legacy database doesn t require this feature, it s better to avoid it. Associations should be to primary keys whenever possible. Most of the new features added to mapping definitions are designed to ease integration for legacy databases. The next feature we cover, dynamic classes, is designed to make development easier.
11.3 Dynamic classes
Throughout this book, we ve discussed using Hibernate with domain models composed of plain old Java objects (POJOs). We can refer to domain models using POJOs as static domain models. The alternative is to use a dynamic domain model. Instead of POJOs, dynamic domain models use Maps to store properties and associations between persistent Maps. Dynamic domain models are useful when the domain model changes rapidly, or when the application is small.
Dynamic classes
The disadvantage of using dynamic domain models is that you lose the explicit strong typing found in static domain models. For instance, when you define a persistent object, you create getters and setters for the various properties:
public class Event { private Long id; private String name; public void setId(Long id) { this.id = id; } public Long getId() { return this.id; } public void setName(String name) { this.name = name; } public String getName() { return this.name; } }
If you try to set a java.lang.String as the id property, you ll get a compile-time error. On the other hand, the Map interface currently only works with Objects, so you can set a java.lang.String as the value of an id property and the error will not be found until you attempt to persist the Map. Despite the lack of strong typing, dynamic domain models can save you a good deal of time and allow you to test new ideas quickly. To declare a persistent Map, the new dynamic-class attribute has been created. Here is an example of mapping a persistent Map:
<dynamic-class entity-name="Event" table="events"> <id name="id" column="id" type="long"> <generator class="native"/> </id> <property name="name" type="string" length="50"/> <property name="startDate" type="date"/> <many-to-one name="location" class="Location" column="location_id"/> </dynamic-class>
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