In this section, we're going to look at a relatively simple (but powerful) example of how data validation / checking can be accomplished when using Iguana (or, indeed, in the context of any application that uses Chameleon and inserts data into a database).

Suppose you are receiving an A04 - Register A Patient message. Every A04 message you are receiving will contain a PID segment that specifies both an internal ID (commonly referred to as a PRK) and an external ID (commonly referred to as an MRN) for the patient that is being registered. These IDs will usually always be the same for the same patient.

Suppose you want your system to ensure that these IDs remain consistent. More specifically, whenever you receive a PID segment for a patient whose MRN number (external ID) is already in your database, you want to make sure that the PRK number you are receiving in the PID segment is exactly the same as the PRK number already recorded for this patient in the database. If not, there is a problem and you want an exception to be raised.

We will cover two approaches to this problem in the next two sections. The simplest approach is the first one, which allows you to use embedded Python inside your VMD file to do this integrity checking.