On Tuesday, March 25, 2014 2:53:42 PM UTC+2, Curtis Maloney wrote:
>
> ps = MyModel.objects.filter(foo__lt=Param('a').prepare()
>
> The result is now a callable that accepts one parameter - "a". To invoke
> the query:
>
> results = ps(a=1000)
>
>
> Clearly it's early days yet - I've written no code. And akaariai has
> pointed out already there's some corners cases which won't work well with
> existing behaviours (e.g. foo=None being silently translated to
> foo__isnull=True), but it's best to get this idea under wider public
> scrutiny earlier, rather than later.
>
I like this style of prepared statements. It is explicit, and
implementation should be achievable without too much added code complexity.
I prefer ps.execute(a=1000) personally, but the exact syntax isn't that
important at this stage.
There will be a couple of corner cases that will be hard to solve. The
problems are around value preparation during .filter() calls and how
certain special values are dealt with. Quickly thinking the value
preparation (basically get_prep_lookup() call) shouldn't be that much of a
problem - it is currently done during .filter() calls, but it should be
possible to defer it to execution time.
The foo=None case is something that likely can't be solved. The problem
here is that foo=None translates to WHERE foo IS NULL, while foo=1
translates to WHERE foo = 1. These are syntactically different queries, and
thus single prepared statement can't handle both of these. There are also
cases where isnull=True/False require different join promotion depending if
True or False is supplied. These again can't be handled.
I am OK for just documenting these corner cases. They aren't something that
should happen too often. The implementation for prepared statements is
relatively straightforward (generate SQL, prepare it once, execute using
given values), but if the corner case needs to be handled the
implementation will be more complex, likely so much more complex that
nobody will implement this feature.
In short: +1 for implementing this with documentation of the corner cases.
- Anssi
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