How to reduce SQL queries in Django using prefetch_related?

Question

I am trying to optimize a Django project (vers. 1.8.6) in which each page shows 100 companies and their data at once. I noticed that an unnecessary amount of SQL queries (especially with contact.get_order_count ) are performed within the index.html snippet below:

index.html:

{% for company in company_list %}
    <tr>
        <td>{{ company.name }}</td>
        <td>{{ company.get_order_count }}</td>
        <td>{{ company.get_order_sum|floatformat:2 }}</td>
        <td><input type="checkbox" name="select{{company.pk}}" id=""></td>
    </tr>
    {% for contact in company.contacts.all %}
        <tr>
            <td>&nbsp;</td>
            <td>{{ contact.first_name }} {{ contact.last_name }}</td>
            <td>Orders: {{ contact.get_order_count }}</td>
            <td></td>
        </tr>
    {% endfor %}
{% endfor %}

The problem seems to lie in constant SQL queries to other tables using foreign keys. I looked up how to solve this and found out that prefetch_related() seems to be the solution. However, I keep getting a TemplateSyntaxError about being unable the parse the prefetch, no matter what parameter I use. What is the proper prefetch syntax, or is there any other way to optimize this that I missed?

I've included relevant snippets of model.py below in case it's relevant. I got prefetch_related to work in the defined methods, but it doesn't change the performance or query amount.

model.py:

class Company(models.Model):
    name = models.CharField(max_length=150)

    def get_order_count(self):
        return self.orders.count()
    def get_order_sum(self):
        return self.orders.aggregate(Sum('total'))['total__sum']

class Contact(models.Model):
    company = models.ForeignKey(
        Company, related_name="contacts", on_delete=models.PROTECT)
    first_name = models.CharField(max_length=150)
    last_name = models.CharField(max_length=150, blank=True)

    def get_order_count(self):
        return self.orders.count()

class Order(models.Model):
    company = models.ForeignKey(Company, related_name="orders")
    contact = models.ForeignKey(Contact, related_name="orders")
    total = models.DecimalField(max_digits=18, decimal_places=9)

    def __str__(self):
        return "%s" % self.order_number

EDIT: The view is a ListView and defines the company_list as model = Company . I altered the view based on given suggestions:

class IndexView(ListView):
    template_name = "mailer/index.html"
    model = Company
    contacts = Contact.objects.annotate(order_count=Count('orders'))
    contact_list = Company.objects.all().prefetch_related(Prefetch('contacts', queryset=contacts))
    paginate_by = 100

Answer 1

Calling the get_order_count and get_order_sum methods causes one query every time the method is called. You can avoid this by annotating the queryset .

from django.db.models import Count, Sum
contacts = Contact.objects.annotate(order_count=Count('orders'), order_sum=Sum('orders'))

You then need to use a Prefetch object to tell Django to use your annotated queryset.

contact_list = Company.objects.all().prefetch_related(Prefetch("contacts", queryset=contacts)

Note that you need to add the prefetch_related to your queryset in the view, it is not possible to call it in the template.

Since you are using ListView , you should be overriding the get_queryset method, and calling prefetch_related() there:

class IndexView(ListView):
    template_name = "mailer/index.html"
    model = Company
    paginate_by = 100

    def get_queryset(self):
        # Annotate the contacts with the order counts and sums
        contacts = Contact.objects.annotate(order_count=Count('orders')
        queryset = super(IndexView, self).get_queryset()
        # Annotate the companies with order_count and order_sum
        queryset = queryset.annotate(order_count=Count('orders'), order_sum=Sum('orders'))
        # Prefetch the related contacts. Use the annotated queryset from before
        queryset = queryset.prefetch_related(Prefetch('contacts', queryset=contacts))
        return queryset

Then in your template, you should use {{ contact.order_count }} instead of {{ contact.get_order_count }} , and {{ company.order_count }} instead of {{ company.get_order_count }} .

Answer 2

在views.py中尝试

company_list = Company.objects.all().prefetch_related("order", "contacts")