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Persisting collections and arrays
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This code prints -1, because a is less than b . If you use the natural ordering of elements, they will typically be ordered from smallest to largest. The last option, an implementation of the Comparator interface, allows custom sorting to be performed on the returned collection. A custom Comparator implementation allows the developer to apply business rules to sorting, or to enhance the natural ordering of a collection.
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Don t let the Comparable and Comparator interfaces confuse you. While they sound similar, they have different functions. Comparable is used to determine if two objects are the same or different. The Comparator interface is used to enforce ordering on a collection of objects.
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The shortcoming of the two approaches that we ve looked at, SQL ordering and Comparator, is that they are static. You can t change them at runtime. Hibernate gets around this by allowing you to apply a filter to a collection. Filters let you sort collections according to some arbitrary field. Suppose you wanted to sort the Attendees for an Event by their phone numbers:
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Set sortedAttendees = session.filter(event.getAttendees(), "order by this.phoneNumber");
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Since you re passing the Set of Attendees, you refer to the collection in the sort clause as this. Applying a filter to a lazy collection does not cause the collection to be initialized, so filters may be used efficiently with very large lazy collections. Now that we ve covered how to manage various types of persistent collections, let s discuss how to create and maintain associations between two persistent classes.
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5.1.6 Bidirectional associations
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The Event class allows you to easily navigate from a parent Event instance to a child Attendee instance. However, suppose you wanted to make the association bidirectional, allowing an Attendee to navigate to its parent Event How would you go about implementing the bidirectional association You must first define the Event property in the Attendee class and provide a many-to-one definition in the Attendee mapping file. Next, the set in the Event must be defined as inverse. Setting the inverse attribute to true informs the Hibernate runtime that the association is bidirectional. The following snippet shows the relevant code from the Attendee class:
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public class Attendee { private Event event; public void setEvent(Event event) { this.event = event; } public Event getEvent() { return this.event; } }
The following snippet shows the relevant code from the mapping file:
<hibernate-mapping package="com.manning.hq"> <class name="Attendee" table="attendees"> <many-to-one column="event_id" class="Event"/> </class> </hibernate-mapping>
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The inverse mapping from the Event class to the Attendee looks like this:
<set name="attendees" inverse="true"> <key column="event_id"/> <one-to-many class="Attendee"/> </set>
This mapping results in the relational schema shown in figure 5.6. Following these three steps, you can now navigate from the Attendee instance to the parent Event instance. Remember that Hibernate only supports bidirectional one-to-many collections for sets and bags. Bidirectional lists, maps, and arrays are not supported, and Hibernate fails to report the error if you have the following in a mapping definition:
<list name="speakers" inverse="true"> </list>
If you try to map a list, map, or array as inverse, the index column of the collection will not be populated when it is persisted.1 Hibernate won t tell you about this mistake it will just fail silently. Next, we ll take a look at mapping many-to-many bidirectional associations.
Many-to-many bidirectional associations
Many-to-many associations may also be bidirectional. In our earlier example, we presented a many-to-many association for Attendees and
events bigint varchar(100) date int ... attendees bigint id ... ... bigint event_id
id name start_date duration ...
Figure 5.6 Bidirectional association for a one-to-many relationship
Okay, if you really want to map an indexed collection as inverse, you can manage the index of the collection yourself. This is explained in detail at www.hibernate.org/116.html#A8.
Collections and custom types
We ll revisit that example and convert it into a bidirectional association. Like the earlier bidirectional association, the Attendee class must have a collection property for the attended Events:
public class Attendee { private Set events; public void setEvent(Event events) { this.events = events; } public Set getEvents() { return this.events; } }
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