goutomroy

@query_debugger
def store_list():

    queryset = Store.objects.all()

    stores = []

    for store in queryset:
        books = [book.name for book in store.books.all()]
        stores.append({'id': store.id, 'name': store.name, 'books': books})

goutomroy

@query_debugger
def store_list_prefetch_related():
  
    queryset = Store.objects.prefetch_related('books')

    stores = []

    for store in queryset:
        books = [book.name for book in store.books.all()]
        stores.append({'id': store.id, 'name': store.name, 'books': books})

goutomroy

@query_debugger
def store_list_expensive_books_prefetch_related():
  
    queryset = Store.objects.prefetch_related('books')

    stores = []
    for store in queryset:
        books = [book.name for book in store.books.filter(price__range=(250, 300))]
        stores.append({'id': store.id, 'name': store.name, 'books': books})

goutomroy

@query_debugger
def store_list_expensive_books_prefetch_related_efficient():

    queryset = Store.objects.prefetch_related(
        Prefetch('books', queryset=Book.objects.filter(price__range=(250, 300))))

    stores = []
    for store in queryset:
        books = [book.name for book in store.books.all()]
        stores.append({'id': store.id, 'name': store.name, 'books': books})

goutomroy

from django.db import models

class Publisher(models.Model):
    name = models.CharField(max_length=300)

    def __str__(self):
        return self.name

class Book(models.Model):
    name = models.CharField(max_length=300)

goutomroy

from django.db import connection, reset_queries
import time
import functools

def query_debugger(func):

    @functools.wraps(func)
    def inner_func(*args, **kwargs):

        reset_queries()

goutomroy

import random
from django.core.management.base import BaseCommand
from apps.bookstore.models import Publisher, Store, Book

class Command(BaseCommand):
    """
    This command is for inserting Publisher, Book, Store into database.
    Insert 5 Publishers, 100 Books, 10 Stores.
    """

goutomroy

# The following will create two QuerySet's, evaluate them, and throw them away 
# because they are not saving the queryset anywhere to reuse them later.
print([e.headline for e in Entry.objects.all()])
print([e.pub_date for e in Entry.objects.all()])

# Following code saves QuerySet in a variable. When it evaluates, 
# it saves the results to its cache(_result_cache). 
queryset = Entry.objects.all()
# evaluation with iteration.

goutomroy

queryset = Entry.objects.all()    
# Evaluated and cached
for each in queryset:
    print(each.headline)

# Using cache from previous evaluation.
for each in queryset:
    print(each.headline)

goutomroy

# You can't use filter to queryset anymore.
queryset = Entry.objects.all()[10:100]
# You can use filter to q1 but not to q2, q3.
q1 = Entry.objects.all()
q2 = q1[1:10]
q3 = q2[1:5]

# saves results to cache of q1
lst1 = [each.blog.id for each in q1]