Java Data Objects

References

The datastore’s representation of a reference to an instance (either a class or interface reference) depends on the identity type defined for the reference’s class. The class’s identity type determines the primary-key (or unique-key) columns of the class’s table. In addition, a class may be mapped to one or more tables. A Java reference is represented in the datastore by a foreign key that refers to the tables associated with the class of the reference. For example, Example 5-1 defined the Movie table. Example 5-2 defines a Role table for the Role class in the com.mediamania.content package. The Role class has a reference, named movie, to the Movie class. On line [1], the Role table defines a foreign key to reference the primary key of the Movie table.

Example 5-2. Foreign key used to reference a primary-key column

CREATE TABLE Role (
    oid     INTEGER,
    name    VARCHAR(20),
    movie   INTEGER,
    PRIMARY KEY(oid),
    FOREIGN KEY(movie) REFERENCES Movie(oid)     [1]
)

Your application does not have to deal with primary and foreign keys; it simply uses standard Java syntax, using the reference to access the object in memory. You also do not need to specify anything specific in the metadata for a reference; its declaration in Java provides all of the necessary information.

JDO supports Java’s polymorphism, allowing a reference to refer to an instance of any subclass of the reference’s declared class. A JDO implementation must be able to determine the type of the instance being referred to, so that ...