Django initial loading of page taking too long - django

Question

Hi I am a beginner at Django and I am working on a project that lists 100 companies in each page along with there contacts and also the amount of items sold. Here is an example:
As you can see the initial loading time of the page is very high. But when I refresh the page it refreshes very fast because I am using caching.
Here are some of my other files:
models.py
from __future__ import unicode_literals
from django.db import models
class Company(models.Model):
name = models.CharField(max_length=150)
bic = models.CharField(max_length=150, blank=True)
def get_order_count(self):
orders = self.orders.count()
return orders
def get_order_sum(self):
orders = Order.objects.filter(company=self)
total_sum = sum([x.total for x in orders])
return total_sum
class Meta:
ordering = ['-id']
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)
email = models.EmailField()
def get_order_count(self):
orders = self.orders.count()
return orders
class Order(models.Model):
order_number = models.CharField(max_length=150)
company = models.ForeignKey(Company, related_name="orders", on_delete=models.CASCADE)
contact = models.ForeignKey(Contact, related_name="orders", on_delete=models.SET_NULL, blank=True, null=True)
total = models.DecimalField(max_digits=18, decimal_places=9)
order_date = models.DateTimeField(null=True, blank=True)
added_date = models.DateTimeField(auto_now_add=True)
modified_date = models.DateTimeField(auto_now=True)
def __str__(self):
return "%s" % self.order_number
views.py
from django.shortcuts import render
# Create your views here.
from django.views.generic import ListView
from mailer.models import Company, Contact, Order
class IndexView(ListView):
template_name = "mailer/index.html"
model = Company
paginate_by = 100
The html
<div class="container">
<table class="table table-borderless">
{% if is_paginated %}
<tr><td>
{% if page_obj.has_previous %}
«
{% endif %}
</td>
<td></td>
<td></td>
<td>
{% if page_obj.has_next %}
»
{% endif %}
</td>
</tr>
{% endif %}
<tr>
<th>Name</th>
<th>Order Count</th>
<th>Order Sum</th>
<th>Select</th>
</tr>
{% 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> </td>
<td>{{ contact.first_name }} {{ contact.last_name }}</td>
<td>Orders: {{ contact.get_order_count }}</td>
<td></td>
</tr>
{% endfor %}
{% endfor %}
</table>
</div>
Is there any way in which I can reduce the initial load time. Please show me an efficient way to solve this problem.

Answer 1

Each {{company.get_order_count}} will hit the DB. Admittedly with a very simple query, but even so, it will slow things down.
You want to annotate the objects with this count. Use
from django.db.models import Count
class IndexView(ListView):
template_name = "mailer/index.html"
model = Company
paginate_by = 100
def get_queryset(self):
return super().get_queryset().annotate( num_orders=Count('orders') )
and replace {{ company.get_order_count }} with {{ company.num_orders }}. This will turn N+1 DB queries into one DB query.
That's the easy one. There's a similar problem with get_order_sum which can almost certainly be solved with another annotation involving the django.db.Sum. Sorry but its late and my stomach is growling and I don't have any confidence that I would get that one right straight off the top of my head.
The cheat sheet on annotation is here. You might also need to look at aggregation.
Oh, and install Django_debug-toolbar in your developer environment. Every time in future it gets slow, you can just click there to see what SQL was executed and how long it took.