I don't have strong opinions on either of the contentious issues in this
discussion, but I do want to weigh-in and say that I think this is an
important and exciting feature.
On Mon, Jun 9, 2014 at 7:48 PM, Josh Smeaton <josh.smea...@gmail.com> 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. It would be slightly better if you could
> check the context from the `prepare()` method (are we preparing order_by or
> not), but then I think we'll run into issues where the prepare breaks for
> 3rd party backends - by checking private data. I really do think we'd be
> better off documenting that __neg__ is reserved by Expressions for
> Ordering, and leave it at that.
>
> I'm surprised how easily expressions slotted in to the order_by - I
> thought it'd take a lot more work. Good stuff I think. Some questions:
>
> - should expressions in order_by support random ordering (?) ? I don't
> think so, but I haven't ever had a need for random ordering so I'm not sure.
> - could we reduce the complexity of the order_by machinery by forcing all
> arguments (internally) to be expressions? `order_by('field_a') ->
> order_by(F('field_a'))` etc
>
> These are open questions, not necessarily directed at Tim.
>
> > I haven't yet implemented the LowerCase and related functions
>
> I think we can build out a handy little library of utility functions like
> Lower and Coalesce that could(should) be included directly in django. But
> it'll also open up, to library developers, the ability to easily develop
> backend specific structures, similar to parts of the
> django.contrib.postgres kickstarter.
>
> Cheers
>
> On Tuesday, 10 June 2014 06:31:28 UTC+10, Tim Martin wrote:
>>
>> On Monday, 9 June 2014 21:28:51 UTC+1, Tim Martin wrote:
>>>
>>>
>>> On Monday, 9 June 2014 20:44:08 UTC+1, Tim Martin wrote:
>>>>
>>>>
>>>> I'll go ahead and try to implement this using __neg__() to invert the
>>>> ordering.
>>>>
>>>>
>>> It's still pretty rough, but there's some code in
>>> https://github.com/timmartin/django/tree/order_by_expressions that
>>> appears to work. In particular, I haven't yet implemented the LowerCase and
>>> related functions (I have one, but it's just a test hack), but that should
>>> just be boiler-plate.
>>>
>>>
>> And I should have noted that I haven't implemented detection of negation
>> in contexts other than ORDER BY. It's not easy to do it cleanly with the
>> simple approach I've got at the moment. The code could just check that
>> get_ordering() returns ASC, but a) that's gross and b) it doesn't solve the
>> problem of having unary negative on a subexpression, which would get
>> ignored.
>>
>> Tim
>>
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Justin Holmes
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845.633.8330
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