Databases Reference
In-Depth Information
C H A P T E R 3
Wiring up Application Logic
If you are an ASP.NET developer, you're probably familiar with the codebehind model introduced by
Microsoft to separate the presentation from the logic layer. In a codebehind page, execution of code is
event-based. You specify blocks of code to execute when a page loads or when a button is clicked.
The APEX logic layer works in a similar fashion. Perhaps the best way to summarize how it works is
this: Page execution is segmented into different chronological events—points at which application logic
can be inserted. For instance, the back end execution of a typical APEX page is segmented into two
major events, Page Rendering and Page Processing, both of which consist of smaller events that can
trigger application logic.
An APEX developer can insert application logic using mostly PL/SQL statements and JavaScript,
both of which can also be generated using the wizards provided by APEX. (In fact, if you don't count
PL/SQL as “code,” it's possible to create an entire bookstore or salesforce application from scratch
without writing a single line of code!)
APEX is different from traditional development tools in that it is geared at churning out business
applications in the shortest amount of time possible. In meeting that goal, APEX enforces a strict
framework that expects business applications to be developed in a certain way. For instance, every form
is expected to display information, provide client-side validation and server-side validation, and perform
back-end processing at specific points in its lifetime. Applications developed on APEX are limited in
nature to this type of flow, and trying to write logic to do something outside this norm may not be as
easy as it would be using a traditional development tool. This means that you can't use APEX to create
games (not great ones, at least), but you can certainly use it to create business applications—especially
database-centric ones—very quickly.
In this chapter, I explore the APEX logic layer in detail and show you how to perform common tasks
in APEX.
3-1. Adding Server-side Validation to Your Form
Problem
You are creating a form to handle patient discharges from a hospital. When nurses fill in the form, they
are required to enter the Social Security number of the patient. Before the record can be created, the
form data must be validated at the back end—the patient's Social Security number must be checked
against a payment history table. If the payment record exists (if the patient has paid for his or her
treatment), the discharge is allowed. If not, the discharge form must display an error message.
