On Mon, 2007-10-08 at 12:14 -0700, Dan Goldner wrote:
> Hello,
> Is it possible to use filter() to query objects based on comparisons
> among attributes of the same object? For example, if my model Task has
> two attributes start_year and end_year and I want to select all tasks
> where start_year was somehow set later than end_year, I would try
> something like
>
> q_backwards = models.Task.objects.filter(end_year__lt =
> start_year)
>
> but that gives a NameError ("global name start_year is not defined").
> Is there any way to reference model object properties on the right
> hand side of the filter argument?
>
> All of the query examples in the docs compare the value of some
> attribute to a constant. If the answer is that you can't compare one
> attribute to another, is there a preferred strategy for this? A model
> method, or custom SQL (hoping to avoid that 'cause I'd have to learn
> SQL) or ....?
At the moment, this requires custom SQL. That is partly why the "where"
parameter to extra() exists.
It will be possible in the future to do a lot of this type of thing more
directly. Russell Keith-Magee had a clever idea a few months back about
how we can make this work with a reasonable syntax -- using something
similar to Q() objects -- and in the near future (certainly before 1.0),
we'll flesh this out and implement it. All the supporting infrastructure
is in place in the queryset-refactor branch and once that's merged we
can fill in the details of his dastardly plan and make it happen.
Regards,
Malcolm
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