Apple WebObjects 5 Manual de usuario descargar pdf (Pagina 29)

CHAPTER 3

Enterprise Objects

Mapping Your Enterprise Objects to Database Tables 29

 Apple Computer, Inc. January 2002

Because of the controller's central mediating role, model objects need not know

about the state and events of the user interface, and view objects need not know

about the programmatic interfaces of model objects.

From the perspective of this paradigm, enterprise objects are model objects.

However, WebObjects also extends the MVC paradigm. Enterprise objects are also

independent of their persistent storage mechanism. Enterprise objects do not need

to know about the database that holds their data, and the database doesn’t need to

know about the enterprise objects.

Mapping Your Enterprise Objects to Database Tables

Enterprise objects make use of a separate file, known as a model, to specify a

mapping between tables in the database and your enterprise-object classes. This is

formally called an entity-relationship (E-R) model. You use EOModeler to create

and maintain these models. With EOModeler you can

■ read the data dictionary from a database to create a default model, which can

then be tailored to suit the needs of your application

■ define data entities that represent the tables in your database

■ define the attributes of each entity; these attributes usually correspond to

columns on a table

■ specify relationships between entities and referential integrity rules for these

relationships

■ generate source code files (enterprise-object classes) for the entities you specify

■ define fetch specifications (queries) that you can invoke by name in your

applications

■ create, modify, or delete tables or databases

A model represents a level of abstraction above the database. The

database-to-objects mapping embodied in a model sets up a correspondence

between database tables and the model’s entities; frequently, table rows map to

instances of the appropriate data entity, as shown in Figure 3-2 (page 30).

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Apple WebObjects 5 Manual de usuario Pagina 29