Hey Ian,
On Tue, 2006-07-11 at 11:59 +1000, Ian Holsman wrote:
> this is what I was thinking of
> from django.db import models
> from django.db.models.query import QuerySet
>
> class SmallQuerySet(QuerySet):
> cache={}
> def get(self, *args, **kwargs):
> cachekey = "%s:%s:%s" % (self.model, str(args), str(kwargs ) )
> if self.cache.has_key( cachekey ) :
> return self.cache[cachekey]
> x= super(SmallQuerySet, self).get(*args,**kwargs)
> self.cache[ cachekey ] = x
> return x
>
> class SmallManager(models.Manager):
> def __init__(self):
> super(SmallManager, self).__init__()
>
> def get_query_set(self):
> """Returns a new QuerySet object. Subclasses can override
> this method
> to easily customise the behaviour of the Manager.
> """
> return SmallQuerySet(self.model)
>
> the aim is to then use this model manager on Site and ContentType.
>
> this of course would mean you would need to 'bounce' your webserver
> if/when you change records in models which use this class.
> but that shouldn't be too often.
>
> is this totally out of line?
> or should I just be using the 'regular' cache here.
Tricky implementation ("tricky" != "bad"), but it looks like it should
work more or less. Don't you need to override every public method of
QuerySet, not just get() (e.g. filter(), order_by()...)?
In a long running process, memory usage is going to be an issue. You are
essentially caching the results of every single query during the runtime
of the process, aren't you? So if I retrieve something that temporarily
sucks in a couple of thousand lines of data and I only need it once, I
pay the memory penalty forever. A "sorted dict" structure that allows
you to remove the oldest entries easily would work around this somewhat
(managing total memory usage wouldn't be easy, but you could manage the
number of cached querysets). I'm a little concerned about whether an
"active" application is just going to end up thrashing the cache a lot
without seeing any real benefit, but the overhead of that should not be
too much. And it's testable once we have an implementation anyway.
I think your idea is probably worthwhile.
Regards,
Malcolm
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