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

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APPENDIX
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The complete Hibernate mapping catalog
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Table schema
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The rooms table holds a foreign key back to the events table. It also has a column, room_color_code, which uniquely identifies the room within the map. Because rooms are entities, they also have their own id column.
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rooms id event_id room_color_code events id
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A.8.2 Many-to-many entity maps
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Maps may also have a many-to-many association with an entity (see figure A.9). In this case, we could easily alter the Room relationship to fit it. This would involve specifying an association table, as well as replacing the one-to-many with a many-to-many. Here s what that class looks like:
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Event
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Room
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Figure A.9 Many-to-many entity map: Event to Room
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public class Event implements Serializable { private Map sharedRooms = new LinkedHashMap(); /** * @hibernate.map table="as_event_to_rooms" * @hibernate.collection-key column="event_id" * @hibernate.collection-index * column="room_color_code" type="string" * @hibernate.collection-many-to-many * class="com.manning.hq.apdxA.Room" column="room_id" */ public Map getSharedRooms() { return sharedRooms; } public void setSharedRooms(Map sharedRooms) { this.sharedRooms = sharedRooms; } }
Maps
Here you re adding another field to Event, called sharedRooms, which is a many-to-many relationship with Rooms. The mapping file looks like this:
<class name="com.manning.hq.apdxA.Event"> <map name="rooms" table="as_event_to_rooms"> <key column="event_id"/> <index column="room_color_code" type="string" /> <many-to-many class="com.manning.hq.apdxA.Room" column="room_id" /> </map> </class>
Table schema
As with all many-to-many relationships, you have an association table with dual foreign keys, which here also holds the map key: room_color_code.
as_event_to_rooms room_id event_id room_color_code
events id
rooms id
A.8.3 Map of components
Maps do not have to contain entities; like any other Hibernate collection, they can hold simple values (like Strings) or component objects. Let s consider an example where our Location has more than just two addresses but potentially a large number of them. You ll add a Map field to the Location object, which will hold Address components, with a key denoting the type of address (Billing, Mailing, Home Office, etc.). Unlike components that are used via the @hibernate.component tag and <component> element, a collection of components needs its own table. This allows any number of components to be mapped back to the
APPENDIX
The complete Hibernate mapping catalog
parent entity. You ll also see a new element, <composite-element>, which will be used instead of <one-to-many> or <many-to-many>. Collections of components are always one-to-many because they are coupled to their parent entity:
public class Location implements Serializable{ private Map addresses = new LinkedHashMap(); /** * @hibernate.map table="addresses" * @hibernate.collection-key column="location_id" * @hibernate.collection-index column="address_type" type="string" * @hibernate.collection-composite-element * class="com.manning.hq.apdxA.Address" */ public Map getAddresses() { return addresses; } public void setAddresses(Map addresses) { this.addresses = addresses; } }
In this example, you can see the new tag, @hibernate.collectioncomposite-element. Here it defines the value of each map entry as an Address component. If you use XDoclet, the generated mapping file should pull the Address properties in the <map> element:
<class name="com.manning.hq.apdxA.Location" table="locations"> <map name="addresses" table="addresses"> <key column="location_id"/> <index column="address_type" type="string"/> <composite-element class="com.manning.hq.apdxA.Address"> <property name="streetAddress" type="java.lang.String" column="street_address" /> </composite-element> </map> </class>
Maps
Table schema
The addresses table holds the foreign key back to the locations table, location_id. The address_type is the unique identifier within the map.
addresses location_id address_type streetAddress locations id
A.8.4 Maps with entity keys
A Map object allows the use of any object as a key. The only constraints are that it must override the equals() and hashCode() methods of the object. In the case of a Hibernate entity, the easiest way to define equals() and hashCode() is to use the primary key.2 Instead of using an unsorted Set of Speakers, you want to store them according to a Room object. Since you want to use Room as a key, you must override both methods as well:
public class Event implements Serializable { private Map speakersByRoom = new LinkedHashMap(); /** * @hibernate.map table="as_event_to_speakers_by_room" * @hibernate.collection-key column="event_id" * @hibernate.index-many-to-many column="room_id" * class="com.manning.hq.apdxA.Room" * @hibernate.collection-many-to-many * class="com.manning.hq.apdxA.Speaker" * column="speaker_id" */ public Map getSpeakersByRoom() { return speakersByRoom; } public void setSpeakersByRoom(Map speakersByRoom) { this.speakersByRoom = speakersByRoom;
The strategy of using the primary key as the foundation for equality, while simple, does have its share of problems, since Hibernate typically generates the keys. For an in-depth discussion of this issues, see Christian Bauer and Gavin King s Hibernate in Action (Manning, 2004).
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