Django中复杂多模型查询的优化

Question

I need an easy control of what products shown on which site, and since there are lots of products, I would also like to set product sites via category, producer and tags. So I show a product on a site if the product OR (its category AND its producer AND one of its tags) are linked to the site. The code I ended up with is the following (simplified):

from django.db.models import Model, ManyToManyField, ForeignKey, BooleanField

class Site(Model):
    enabled = BooleanField()

class Producer(Model):
    sites = ManyToManyField(Site)

class Category(Model):
    sites = ManyToManyField(Site)

class Tag(Model):
    sites = ManyToManyField(Site)

class Product(Model):
    producer = ForeignKey(Producer)
    category = ForeignKey(Category)
    tags     = ManyToManyField(Tag)
    sites    = ManyToManyField(Site)

    def get_site_ids(self):
        if self.sites.exists():
            return list(self.sites.values_list('id', flat=True))
        else:
            p = list(self.producer.sites.values_list('id', flat=True))
            c = list(self.category.sites.values_list('id', flat=True))
            t = sum([list(tag.sites.values_list('id', flat=True)) for tag in self.tags.all()], [])
            ids = set.intersection(set(p), set(c), set(t))
            return ids

My question is how can I optimize get_sites method? Is it possible to join some of the queries?

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Update: I am interested just in site ids which will be indexed and stored by search engine.

Answer 1

You should try and do everything you want to do on the database level if possible. Something like this should get you started.

from django.db.models import Sum
self.objects.select_related("producer__sites",
                            "category__sites",
                            "tags__sites",
                            "sites").annotate(tag_count=Sum("tags__sites__id"))

select_related returns the Product model with the producer, category, tags and sites references filled. This also includes the sites reference in producer, category and tags. This make Django do less queries to the database.

annotate adds an attribute to the returned Model instance named tag_count that has the Sum of tags_ sites _id in it.

The Django QuerySet Reference might be a good help because I don't know the exact nature of what you want to have returned.

Answer 2

Yes you can reduce the number of queries in Django using select_related and/or prefetch_related .

However, the select_related only works for foreign key and one-on-one relationships. Since you have ManyToMany in your Product model, you should use prefetch_related to reduce your queries in Product.get_sites().