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

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 <jrschneider83@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.rachum@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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