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]](img/note.gif) | 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”.
Tables 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 which understands the following attributess:
name: The name for this table.
schema: The schema for this table
catalog: The catalog for this table
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
{
...
}