android java qr code generator The complete Hibernate mapping catalog in Java

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The complete Hibernate mapping catalog
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A.6 Set: many-to-many
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Like all collections, Sets can handle both one-to-many and many-tomany associations. From the mapping file perspective there is not much difference, with only one element (or XDoclet tag) being swapped out. From the database perspective, it s considerably different, since it requires the use of an association table to hold the foreign keys. When persisting and querying, clients of the model class don t need to worry about this extra table, though, because Hibernate manages it under the covers. But you do need to consider this table when writing the mappings. Many-to-many Sets can be bidirectional; see figure A.6.
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Figure A.6 Many-to-many set: Event to Attendee
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A.6.1 Unidirectional
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The unidirectional set consists of a Set field on one of the objects. In this case, our Event has a many-to-many relationship with Attendees. Attendees can go to many Events, and each Event has many Attendees:
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public class Event implements Serializable { private Set speakers = new LinkedHashSet(); /** * @hibernate.set table="as_event_to_attendee" * @hibernate.collection-key column="event_id" * @hibernate.collection-many-to-many * class="com.manning.hq.apdxA.Attendee" * column="attendee_id" */ public Set getAttendees() { return attendees; } public void setAttendees(Set attendees) { this.attendees = attendees; } }
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Set: many-to-many
As you can see, the <one-to-many> tag has been replaced with a <manyto-many> tag. In addition, the event_id is not stored in the attendee table, but in an association table, here called as_event_to_attendee.1 The mapping file looks like the following:
<class name="com.manning.hq.apdxA.Event" table= events > <set name="attendees" table="as_event_to_speaker"> <key column="event_id" /> <many-to-many class="com.manning.hq.apdxA.Attendee" column="attendee_id" /> </set> </class>
Table schema
For a many-to-many relationship, three tables are needed: one table for each of the entities, Event and Attendee, and one association table for the many-to-many relationship.
events id attendees id as_event_to_speaker event_id attendee_id
A.6.2 Bidirectional
The bidirectional many-to-many Set puts another set on the object on the opposite end. In our current example, the Attendee would have a Set of Events. A bidirectional many-to-many set is going to result in quite a few queries when loading, especially if the number of Events and Attendees is large. So do some performance testing with the show_sql parameter on to check it. The following code is the modified Attendee with its Set of Events:
The naming convention here of the association table, using as_ for a prefix, then the name of the tables being joined here, is strictly ours. We find it helps to distinguish between entity tables as the strictly associative ones.
The complete Hibernate mapping catalog
/** * @hibernate.class table="attendees" */ public class Attendee implements Serializable { private Set events = new LinkedHashSet(); /** * @hibernate.set inverse="true" table="as_event_to_attendee" * @hibernate.collection-key column="attendee_id" * @hibernate.collection-many-to-many class="com.manning.hq.apdxA.Event" column="event_id" */ public Set getEvents() { return events; } public void setEvents(Set events) { = events; } }
When dealing with a bidirectional many-to-many relationship, you have to mark one end of it as inverse. Which end you mark is your choice. By marking the set of Events on Attendee as inverse, you re telling Hibernate that the Event object is responsible for maintaining the relationship. Any changes made to the inverse end that aren t also made to the non-inverse end won t be saved. The above code is virtually identical to that for the Event.attendees field, except that the columns are reversed. The attendee_id is now the key column and the outward foreign key is now event_id. Here s what the Attendee.hbm.xml file will contain:
<class name="com.manning.hq.apdxA.Attendee" table="attendees"> <set name="events" table="as_event_to_attendee" inverse= true > <key column="attendee_id"/> <many-to-many class="com.manning.hq.apdxA.Event" column="event_id" /> </set>
Most of the remaining collection types will follow this pattern, allowing both one-to-many and many-to-many associations.
A.7 Lists
Hibernate allows you to store Lists of entities, using a <list> element in the mapping file. The contract of java.util.List is that it maintains the order of insertion but has no guarantees that it won t contain duplicate elements. For Hibernate, this means it needs to store the index of each element in the database in its own column, which must be an integer type.
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