c# create and print barcode Building collections in Java

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Building collections
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9.4.1 One-to-many: a kicking set of Speakers
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Our next example is a remake of the original Event and Speaker example from chapter 5. Recall that an Event has a one-to-many relationship with Speakers, stored as a Set property on Event. To build a collection with XDoclet, you can t just use a single tag, as we did with the @hibernate.id and @hibernate.many-to-one tags. Instead, you have to use multiple cooperating tags, such as the @hibernate.property and @hibernate.column tags. One of the things that makes XDoclet generation difficult is knowing which tags to use together, something that the XDoclet tag reference documentation doesn t currently make clear. Typically you will need at least three tags, one for each nested element and one for the collection element. The net result is something that is easier to demonstrate than it is to explain, so let s demonstrate first. Modify the Event class as shown in listing 9.4 to give it a set of Speakers and the corresponding XDoclet tags. Listing 9.4 Event with a set of Speakers
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import java.util.LinkedHashSet; import java.util.Set; /** * @hibernate.class table="events" */ public class Event implements Serializable { // Other properties omitted. private Set speakers = new LinkedHashSet(); // Other getter/setters omitted. Declares the Declares the /** collection type foreign key * @hibernate.set cascade="save-update" from Speaker to Event * @hibernate.collection-key column="event_id" * @hibernate.collection-one-to-many class="com.manning.hq.ch09.Speaker" Declares the collection a one*/ to-many along with the class public Set getSpeakers() { return speakers; } public void setSpeakers(Set speakers) { this.speakers = speakers; } }
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Hibernating with XDoclet
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Needing three tags to generate the mapping for just one property may seem a bit overwhelming. This code tells XDoclet that the collection is a Set, which column on the Speaker object is the foreign key back to Event, and that it s a one-to-many (as opposed to a many-to-many) relationship between Events and Speakers. XDoclet cannot inspect the code to determine what class of object the speaker s collection holds, so you must explicitly tell it that it holds a set of com.manning.hq.ch09.Speaker instances. If you rerun the Ant generate-hbm task, the task will add the following fragment to the Event.hbm.xml file:
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<set name="speakers" lazy="false" inverse="false" cascade="save-update" sort="unsorted" > <key column="event_id" ></key> <one-to-many ="com.manning.hq.ch09.Speaker" /> </set>
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Here the method to XDoclet s madness is made clear. The Rule of XDoclet collections is: To generate collections, every element requires a single XDoclet tag. If you know what the end resulting mapping fragment looks like, you should be able to reasonably determine which tags you need to generate it. For our previous example you needed three elements, as shown in table 9.7, which offers a line-by-line comparison.
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Table 9.7 Matching Hibernate mapping elements to XDoclet tags Mapping Element <set /> A nested <key /> A nested <one-to-many /> XDoclet Tag @hibernate.set @hibernate.collection-key @hibernate.collection-one-to-many
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Given the comparison in table 9.7, what tag do you think you would need if you wanted to change the Speaker to Event one-to-many
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Building collections
relationship into a many-to-many relationship Assume that the end result is an element like this:
<set name="speakers" table="as_event_to_speaker"> <key column="event_id" ></key> <many-to-many class="com.manning.hq.ch09.Speaker" column="speaker_id" /> </set>
If you guessed @hibernate.collection-many-to-many, you d be absolutely correct. And the prize is that you don t need to scour the XDoclet tag reference using trial and error to guess which one you need to use.5
9.4.2 The @hibernate.set tag
Having seen the @hibernate.set tag in action, let s step back and take a look at its common attributes. Since a set supports both many-tomany and one-to-many, some of the attributes are used for each of those cases, as table 9.8 shows.
Table 9.8 Common @hibernate.set attributes Attribute cascade Description Specifies how cascading operations should be handled from parent to child. For the many-to-many association only, contains the association table name for joins. Default None. Acceptable values include all, none, saveupdate, all-delete-orphan, and delete. For a many-to-many, it uses the name of the field.
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