Well, the Desk model you provided is blank, but I'll believe you that
there's a favorite_or_nearby_chairs attribute. ;-)
Should be relatively simple. Just add a .select_related('nearby_desks') to
your existing query and that should pull in the associated Desk object in a
single query. You can also substitute in prefetch_related(), although
you'll still have two queries at that point.
If you are trying to profile your site, I would recommend the
Django-debug-toolbar. That should tell you whether or not that query set is
the culprit.
-James
On Feb 25, 2015 1:28 PM, "Ram Rachum" <[email protected]> wrote:
> Hi James,
>
> I've read the docs but I still couldn't figure it out. My queryset works
> great in production, I'm trying to optimize it because our pageloads are
> too slow. I know how to use querysets in Django pretty well, I just don't
> know how to use `Prefetch`.
>
> Can you give me the solution for the simplified example I gave? This might
> help me figure out what I'm not understanding. One thing that might be
> unclear with the example I gave, is that I meant I want to get a queryset
> for `Desk` where every desk has an attribute names `favorite_or_nearby_chairs`
> which contains the queryset of chairs that I desrcibed, prefetched.
>
>
> Thanks,
> Ram.
>
> On Wed, Feb 25, 2015 at 11:18 PM, James Schneider <[email protected]
> > wrote:
>
>> I assume that you are talking about the select_related() and
>> prefetch_related() queryset methods?
>>
>> https://docs.djangoproject.com/en/1.7/ref/models/querysets/#select-related
>>
>> https://docs.djangoproject.com/en/1.7/ref/models/querysets/#prefetch-related
>>
>> Both of those sections have excellent examples, and detail what the
>> differences are (primarily joins vs. separate queries, respectively).
>>
>> For better help, you'll need to go into more detail about the queries you
>> are trying to make, what you've tried (with code examples if possible), and
>> the results/errors you are seeing.
>>
>> In general, I would try to get an initial queryset working and gathering
>> the correct results first before looking at optimizations such as
>> select_related(). Any sort of pre-fetching will only confuse the situation
>> if the base queryset is incorrect.
>>
>> -James
>>
>> On Wed, Feb 25, 2015 at 12:05 PM, cool-RR <[email protected]> wrote:
>>
>>> Hi guys,
>>>
>>> I'm trying to solve a problem using the new `Prefetch` but I can't
>>> figure out how to use it.
>>>
>>> I have these models:
>>>
>>> class Desk(django.db.models.Model):
>>> pass
>>>
>>> class Chair(django.db.models.Model):
>>> desk = django.db.models.Foreignkey('Desk', related_name='chair',)
>>> nearby_desks = django.db.models.ManyToManyField(
>>> 'Desk',
>>> blank=True,
>>> )
>>>
>>> I want to get a queryset for `Desk`, but it should also include a
>>> prefetched attribute `favorite_or_nearby_chairs`, whose value should be
>>> equal to:
>>>
>>> Chair.objects.filter(
>>> (django.db.models.Q(nearby_desks=desk) |
>>> django.db.models.Q(desk=desk)),
>>> some_other_lookup=whatever,
>>> )
>>>
>>> Is this possible with `Prefetch`? I couldn't figure out how to use the
>>> arguments.
>>>
>>>
>>> Thanks,
>>> Ram.
>>>
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