Blog of Jeff A blog about programming and random other things.

Beginning to Code

Okay, now we're ready to start writing code! In normal Django development, you want to plan out what data your database will contain, this is important. This is due to one current flaw in Django: that Django lacks a schema evolution system. Django can only reset or create tables, but not modify existing ones.

Once you've planned out your data structures for your database, you can start creating models. Models describe how your data is stored in the database and any extra metadata your application needs to be aware of but the underlying database system may or may not be storing.

For this tutorial, I've already have a set of models designed (with some consideration for features not implemented in this tutorial series). We're going to have 4 model objects:

• Room: this contains room information, such as name, owner, etc.
• Message: this contains the actually message a user sends.
• Membership: this model associates a Room with a User (aka, we know that user can view this chat room)
• Attachment: this contains file upload information that may be associated to a particular message.

Model definitions are fairly easy to make. Let's make the Room model:

from django.db import models
import datetime
class Room(models.Model):
     name = models.CharField(max_length=200)
     created_at = models.DateTimeField('created at', default=datetime.datetime.now)
     def __unicode__(self):
         return u'%s' % self.name

Most of the code is simple. The first two lines imports the Django Model class that Django provides (which makes model definitions extremely simple) and we import the native python datetime module for, you guessed it, time functionality.

In the Room class definition we simply define properties to that tells Django what type of data each property stores. These properties are mapped as fields in the database.

For example, the name property we set stores a string (char or varchar in database terms) with a max length of 200 characters. The other two properties are just as readable. The first parameter for nearly all Field functions is a human-readable name of the field. In the case for the name property, Django will just use the field name. Also, the __unicode__ method provides a display-friendly output of a particular model (which we're outputting the room name). The friendly names are for Django's automatic admin.