In the top-down mapping approach, you generate a default database
schema and mapping based on one or more existing enterprise beans.
In EJB 2.x, your mapping and schema files make up a backend for
EJB 2.x projects. You can have multiple backend folders for each project (for
example, one DB2® and
one Oracle backend). Only one database backend is used as the default, but
you can define as many as you need. So, the first page in the EJB 2.x mapping
wizard asks you to specify whether you want to create a new backend, in which
case you then proceed as outlined here, or you can use an existing backend,
like for meet-in-the-middle or bottom-up1 where only the schema
exists in the backend (no map file). If you have previously generated a mapping,
you can choose to create and map unmapped elements or open the mapping editor
to manually make changes.
To generate a schema and map from existing
enterprise beans using the top-down mapping approach:
- In the J2EE perspective, right-click the EJB project and select . This option will not be available if you have the mapping
editor open.
- For EJB 2.x projects, select whether to Create a new
backend folder or to Use an existing backend folder.
If you choose to use an existing backend, and if you have previously generated
a mapping, you must choose one of the following options at this point:
- Create and map unmapped elements: use this
option if you have made changes to the backend or to the enterprise beans
and want to create the new elements and update the map accordingly.
- Open mapping editor on selected backend map:
use this option to open the mapping editor, where you can make manual updates
to the mapping.
Click Finish to perform the action and exit the
wizard.
- If you choose to create a new backend folder, click Next to
continue creating your initial top-down mapping.
- Select Top Down, and click Next.
- Select a Target Database.
- Type the Database name and Schema name.
- Optional: You can select one or both of the following
check boxes:
- Generate DDL
- Add a column for collision detection: use this
option for EJB 2.x CMP beans to add a column to your relational table. This
column is reserved to determine if a record has been updated. For more information
about this collision detection column, refer to the related topic about adding
a column for collision detection.
- WebSphere 3.x Compatible - only select this
option if you are migrating from WebSphere Application Server 3.x deployments.
- Click Finish to create the top-down mapping,
or click Next to further define inheritance.
- If inheritance exists, select additional enterprise beans from
the list to add a leaf table for each checked bean that joins with the parent
table, then click Finish.
The EJB mapping tools create tables, columns, and constraints relating
to a CMP bean and its fields, as well as a map of each CMP field to a column.
You can then make any required changes to these maps using the Mapping editor.
Use the relational database tools to modify the tables and columns.