Re: Can the new `Prefetch` solve my problem?

got it, so you want to prefetch but not all chairs.

I will def follow this thread to see the possibilities of Prefetch :)

On 02/26/2015 12:52 PM, Ram Rachum wrote:

There may be a big number of chairs, and I don't want all the chairs prefetched. I want to have the database filter them according to the queryset I specified in a single call, I don't want to filter them in Python or make a new call to filter them.

Thanks,

Ram.

On Thu, Feb 26, 2015 at 1:48 PM, aRkadeFR <contact@arkade.info> wrote:

I may not have completely understand your problem, but
why not prefetching all the chairs? and then with the (new)
attribute favorite_or_nearby_chairs loading only the favorite
or nearby one?

like:
@property
def favorite_or_nearby_chairs(self):
    for chair in self.chair_set.all():
          #filter...
          ans += ...
    return ans

It will only hit the DB once thanks to the first join of desk
<-> chair.

On 02/26/2015 11:28 AM, cool-RR wrote:

James, you misunderstood me.

There isn't supposed to be a `favorite_or_nearby_chairs` attribute. That's the new attribute I want the prefetching to add to the `Desk` queryset that I need. Also, I don't understand why you'd tell me to add a `.select_related('nearby_desks')` to my query. Are you talking about the query that starts with `Chair.objects`? I'm not looking to get a `Chair` queryset. I'm looking to get a `Desk` queryset, which has a prefetched attribute `favorite_or_nearby_chairs` which contains the `Chair` queryset I wrote down.

Thanks,

Ram.

On Thursday, February 26, 2015 at 6:02:15 AM UTC+2, James Schneider wrote:

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" <r...@rachum.com> 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 <jrschn...@gmail.com> 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 <ram.r...@gmail.com> 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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