Efficient way to perform many queries

[9dev...@gmail.com]

I am writing application similar to django-taggit and I was wondering about its performance.

Example for list of items:

{% for item in items %}

    {{ item.name }}

    {% for tag in item.tags.all %}
        {{ tag }}
    {% endfor %}

{% endfor %}

Above code creates massive amount of separated SQL queries as can be observed in Django Toolbar (click on image to enlarge).

Is there any way to make it more efficient? I was thinking about merging these queries into one big. Thanks in advance for any tips.

[Collin Anderson]

you could try in your view:

items = items.prefetch_related('tags')

[9dev...@gmail.com]

Thank you.

What if I'm using GenericForeignKey in Tags model? Can I still use it?

[Sergiy Khohlov]

good question ! 

1) I like to use some optimization at view side not template 

2) if i need something like your code I using optimization.

 Something like that : 

{% with items = mymodels.items.all  %}

{% for item in items %}

    {{ item.name }}

    {% for tag in item.tags.all %}

        {{ tag }}

    {% endfor %}

{% endfor %}

{% endwith %}

 

First records means get ALL record to the variable which should be cached and used sometimes later 

3) Anyway  i dont like  "object.all "

I like to retrieve minimum of the field for decreasing database loading 

Many thanks,

Serge


+380 636150445
skype: skhohlov

On Fri, Aug 15, 2014 at 5:46 PM, Collin Anderson <cmawe...@gmail.com> wrote:

you could try in your view:

items = items.prefetch_related('tags')

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[9dev...@gmail.com]

prefetch_related() works great.

How about prefetching some computed value instead of records?

{% for item in items %}

    {{ item.name }}

    {{ item.score }}

{% endfor %}

I want to prefetch 'score' for every item, where 'score' would be equivalent to:

score = Votes.objects.all().count() - Votes.objects.filter(value=False).count()

Is there any way to prefetch such query?

Code below prefetches all data for Vote records. How can it be changed to prefetch only the score?

Item.objects.all().prefetch_related('votes')

[Collin Anderson]

I think this should work:

Item.objects.exclude(votes__value=False).annotate(score=Count('Votes'))

[9dev...@gmail.com]

Thank you, it works perfectly :)