Question

I have a following model:

class Car(models.Model):     make = models.CharField(max_length=40)     mileage_limit = models.IntegerField()     mileage = models.IntegerField() 

I want to select all cars where mileage is less than mileage_limit, so in SQL it would be something like:

select * from car where mileage < mileage_limit; 

Using Q object in Django, I know I can compare columns with any value/object, e.g. if I wanted to get cars that have mileage say less than 100,000 it would be something like:

cars = Car.objects.filter(Q(mileage__lt=100000)) 

Instead of a fixed value I would like to use the column name (in my case it is mileage_limit). So I would like to be able to do something like:

cars = Car.objects.filter(Q(mileage__lt=mileage_limit)) 

However this results in an error, since it is expecting a value/object, not a column name. Is there a way to compare two columns using Q object? I feel like it would be a very commonly used feature and there should be an easy way to do this, however couldn’t find anything about it in the documentation.

Note: this is a simplified example, for which the use of Q object might seem to be unnecessary. However the real model has many more columns, and the real query is more complex, that’s why I am using Q. Here in this question I just wanted to figure out specifically how to compare columns using Q.

EDIT

Apparently after release of Django 1.1 it would be possible to do the following:

cars = Car.objects.filter(mileage__lt=F('mileage_limit')) 

Still not sure if F is supposed to work together with Q like this:

cars = Car.objects.filter(Q(mileage__lt=F('mileage_limit'))) 

Answer 1

You can’t do this right now without custom SQL. The django devs are working on an F() function that would make it possible: #7210 – F() syntax, design feedback required.