Working with Object-relational Descriptors

The object-relational paradigm extends traditional relational databases with object-oriented functionality. Oracle, IBM DB2, Informix, and other DBMS databases allow users to store, access, and use complex data in more sophisticated ways.

The object-relational standard is an evolving standard mainly concerned with extending the database data structures and the SQL language (SQL 3).

The new features include:

Structures or Object-types can be defined and stored on the database
Collections/Arrays can be defined and stored on the database
Structures/Object-types can have system-generated ObjectIDs
Structures/Object-types can reference other structures through References or aggregation
SQL 3, an extension to the SQL language that supports querying and manipulating the new object-types

Coinciding with object-relational changes, most database vendors are also extending their server architectures to support features such as:

Embedded server-side Java VMs
Java stored procedures
CORBA, HTML and EJB support in the database

This section describes how the object-relational features affect OracleAS TopLink descriptors and mappings. The server architecture changes are discussed in the Oracle Application Server TopLink Application Developer's Guide.

Effect on OracleAS TopLink

Object-relational databases introduce several new features that allow more complex data to be stored and accessed. One advantage of object-relational databases is that the differences between the object model and data model can be reduced to the point that the two are almost identical. Although this makes the object-relational mapping process easier, it does not reduce the need for a persistence framework such as OracleAS TopLink. Although the JDBC standard has been improved to take advantage of object-relational features in JDBC 2.0, it still remains a low-level database interface. On top of JDBC, frameworks such as OracleAS TopLink can provide applications with much more sophisticated functionality, including units of work, identity maps, expressions, querying, complex mappings, three-tier and enterprise application support.

OracleAS TopLink provides object-relational support through a new type of descriptor object and several new types of mappings. See Chapter 7, "Understanding Object-Relational Mappings" for more information.

Databases OracleAS TopLink Supports

OracleAS TopLink supports any JDBC 2.0 driver that complies with JDBC's 2.0 object-relational extensions. Contact your database and JDBC vendor to determine which object-relational features they support.

Defining Object-Relational Descriptors

The OracleAS TopLink Mapping Workbench does not currently support the object-relational descriptor and mappings. Support will be added to the OracleAS TopLink Mapping Workbench in future releases.

You should be able to import most of the simple object-relational table structures into OracleAS TopLink. In addition, you can define the standard non-object-relational descriptor properties and mappings. You can use amendment methods to add any object-relational mappings and features to the descriptors.