Modeling Challenges In Django

Supporting code and slides for a talk originally given at DjangoCon-US, September 2010 (in Portland, Oregon, USA).

Short Description

How would you model players, umpires and coaches in baseball data when the same person can switch roles over the course of their life? How about servers in racks with power boards attached (and cords running across the room to remote boards)? Here is one approach to create minimal and well-performing models for such real-life situations.

Abstract

The slightly over-simplified but useful rule of thumb when creating database schema is “normalize until it hurts, [then] denormalize until it works.” If only people didn’t skip the first step so often. Using a data modeling layer, such as Django's models, doesn't absolve the system architects from the need to create good design. It also doesn't require them to do so, since you can get away with a lot of sub-optimality with many data sets.

The real difficulty here, though, is that the trade-off between text-book ideal modeling and easy to use is difficult to judge and takes practice to develop.

This talk will walk through some interesting cases of model design that I've encountered recently. I'll explain how I approached the problem and what we ended up with. These will include:

• Modeling people who might simultaneously play different roles in the system. For example, a person who was a baseball player and then became a coach — each role has different attributes attached to it.
• Modeling what appears to be a triangular dependency relationship with minimal redundancy in the data description and without needing really long query filters to access things.
• Handling date ranges (or other measured data) of different degrees of accuracy and precision.

This isn't a presentation on theoretical database design. Rather, concrete examples of creating such designs and guiding the decisions by what might work best in the final Django code. Hopefully, by listening to one person's approach (mine!), people faced with similar challenges will have another possible attack method in their toolbox.

Setup

Everything is configured to create an SQLite database and an automatic admin user. Simply run:

python manage.py syncdb --noinput

The admin user has username and password both set to "admin" (with the quotes).

Tour of the code

There are two applications included in this code package, providing models and a brief amount of supporting code for the two cases covered in the presentation.

The dates/ application is a pair of simple models and is the easier of the two cases. The sports/ application is a tighter group of related models, that has been reduced (over the course of the presentation) to something manageable. The admin presentation for these models contains one enhancement: the team display page includes extra information about the current members and coaches (have a look in the templates directory to see how that is accomplished).

By default, both applications will be installed with sample data and are viewable via Django's admin interface.

Good luck!

Malcolm Tredinnick (Sydney, Australia)