Django DB Calculations

What

This is a way for doing calculations in the database, and having them magically appear as attributes on your models.

Why

Django's extra isn't DRY.

Usage

You'll need Django 1.2. Add the custom manager to your model, like so

from django.db import models
from db_calculations import CalculationManager

class MyModel(models.Model):
    ...
    objects = CalculationManager()

Now you can use the calculate method, using the builtin F objects to build your expression

Example

>>> from basic.models import Number
>>> from django.models import F
>>> Number.objects.create(x=2, y=3)
>>> Number.objects.calculate(z=F('x')+F('y'))[0].z
... 5.0

>>> Number.objects.calculate(z=F('x')-F('y'))[0].z
... -1.0

>>> Number.objects.calculate(z=F('x')*F('y'))[0].z
... 6.0

>>> Number.objects.calculate(z=F('x')/F('y'))[0].z
... 0.66666666666666663

>>> Number.objects.calculate(z=F('x')+F('y')).calculate(z_squared=F('z')*F('z'))[0].z_squared
... 25.0

Warning

The current state is more of a proof of concept than any sort of thing you might want to use for your project. It pretty much won't work outside of basic operations. In fact, it will go pretty crazy once you start to do anything fancy.