On Tuesday 10 June 2014 02:48:14 Josh Smeaton wrote:
> > However, I think having some special case code in filter(), annotate()
>
> and anything else that takes expressions would be OK
>
> I strongly disagree with this. Expressions are context insensitive at the
> moment, which means they can be used anywhere. If we start externalising
> behaviour based on the context of use, it'll lead us to some very confusing
> code and a tonne of special cases.
+1 Josh.
However, per the __neg__ issue, I think you're heading in the wrong direction:
ordering shouldn't be a feature of an expression, because an expression with
ordering cannot be used the same way: It cannot be combined with other
expressions (because the ordering of the result is intuitively ambiguous), it
shouldn't be used for comparisons etc.
So, I think, in terms of implementation (and I'm doing this in horrible style,
without looking at your actual patches -- sorry if I'm way off), once you apply
ordering to an expression, you should get a different-typed object. Indeed, a
wrapper would be suitable, but it should no longer be an expression IMO.
And in terms of API as well -- the hints should be clear that this is different
from normal expression creation. Both "-Expression" and "Desc(Expression)"
look like other expressions, so I think they're suboptimal. I'd prefer
something like "Expression.desc()" which reads nicely:
Model.objects.filter(...).order_by( F('fld_a').desc(),
F('fld_b')).asc() )
Model.objects.filter(...).order_by( (F('fld_a')+F('fld_b')).desc() )
(desc as a property instead of method would read nicer but feels less "right"
as Python).
My 2 cents,
Shai.
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