Object-relational mapping is the process of mapping entities to relational database tables. In EJB persistence, you perform object/relational mapping through mapping metadata. Mapping metadata uses annotations to describe how to link your object model to your relational model.
Note | |
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Kodo offers tools to automate mapping and schema creation. See Chapter 7, Mapping in the Reference Guide. |
Throughout this chapter, we will draw on the object model introduced in Chapter 5, Metadata. We present that model again below. As we discuss various aspects of mapping metadata, we will zoom in on specific areas of the model and show how we map the object layer to the relational layer.
All mapping metadata is optional. Where no explicit mapping metadata is given, EJB 3 persistence uses the defaults defined by the specification. As we present each mapping throughout this chapter, we also describe the defaults that apply when the mapping is absent.
The Table
annotation specifies the table
for an entity class. If you omit the Table
annotation, base entity classes default to a table with their
unqualified class name. The default table of an entity subclass
depends on the inheritance strategy, as you will see in
Section 12.6, “Inheritance”.
Table
s have the following properties:
String name
: The name of the table.
Defaults to the unqualified entity class name.
String schema
: The table's schema. If you
do not name a schema, EJB uses the default schema for the
database connection.
String catalog
: The table's catalog. If
you do not name a catalog, EJB uses the default catalog for the
database connection.
UniqueConstraint[] uniqueConstraints
: An
array of unique constraints to place on the table.
We cover unique constraints below. Defaults
to an empty array.
The equivalent XML element is table
. It has
the following attributes, which correspond to the annotation
properties above:
name
schema
catalog
The table
element also accepts nested
unique-constraint
elements representing unique constraints.
We will detail unique constraints shortly.
Sometimes, some of the fields in a class are mapped to secondary
tables. In that case, use the class' Table
annotation to name what you consider the class' primary table. Later,
we will see how to map certain fields to other tables.
The example below maps classes to tables according to the following
diagram. The CONTRACT
, SUB
, and
LINE_ITEM
tables are in the CNTRCT
schema; all other tables are in the default schema.
Note that the diagram does not include our model's Document
and Address
classes. Mapped
superclasses and embeddable classes are never mapped to tables.
Example 12.1. Mapping Classes
package org.mag; @Entity @IdClass(Magazine.MagazineId.class) @Table(name="MAG") public class Magazine { ... public static class MagazineId { ... } } @Entity @Table(name="ART") public class Article { ... } package org.mag.pub; @Entity @Table(name="COMP") public class Company { ... } @Entity @Table(name="AUTH") public class Author { ... } @Embeddable public class Address { ... } package org.mag.subscribe; @MappedSuperclass public abstract class Document { ... } @Entity @Table(schema="CNTRCT") public class Contract extends Document { ... } @Entity @Table(name="SUB", schema="CNTRCT") public class Subscription { ... @Entity @Table(name="LINE_ITEM", schema="CNTRCT") public static class LineItem extends Contract { ... } } @Entity(name="Lifetime") public class LifetimeSubscription extends Subscription { ... } @Entity(name="Trial") public class TrialSubscription extends Subscription { ... }
The same mapping information expressed in XML:
<entity-mappings xmlns="http://java.sun.com/xml/ns/persistence/orm" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence/orm orm_1_0.xsd" version="1.0"> <mapped-superclass class="org.mag.subscribe.Document"> ... </mapped-superclass> <entity class="org.mag.Magazine"> <table name="MAG"/> <id-class="org.mag.Magazine.MagazineId"/> ... </entity> <entity class="org.mag.Article"> <table name="ART"/> ... </entity> <entity class="org.mag.pub.Company"> <table name="COMP"/> ... </entity> <entity class="org.mag.pub.Author"> <table name="AUTH"/> ... </entity> <entity class="org.mag.subcribe.Contract"> <table schema="CNTRCT"/> ... </entity> <entity class="org.mag.subcribe.Subscription"> <table name="SUB" schema="CNTRCT"/> ... </entity> <entity class="org.mag.subscribe.Subscription.LineItem"> <table name="LINE_ITEM" schema="CNTRCT"/> ... </entity> <entity class="org.mag.subscribe.LifetimeSubscription" name="Lifetime"> ... </entity> <entity class="org.mag.subscribe.TrialSubscription" name="Trial"> ... </entity> <embeddable class="org.mag.pub.Address"> ... </embeddable> </entity-mappings>