susmithagudapati

def application(environ, start_response):
    start_response("200 OK", [("Content-Type", "text/plain")])
    return b"Hello, World"

susmithagudapati

async def application(scope, receive, send):
    await send({"type": "http.response.start", "status": 200, "headers": [(b"Content-Type", "text/plain")]})
    await send({"type": "http.response.body", "body": b"Hello World"})

susmithagudapati

from asgiref.sync import async_to_sync

sync_function = async_to_sync(async_function)

@async_to_sync
async def async_function(...):
    ...

susmithagudapati

from asgiref.sync import sync_to_async

async_function = sync_to_async(sync_function)
async_function = sync_to_async(sensitive_sync_function, thread_sensitive=True)

@sync_to_async
def sync_function(...):
    ...

@sync_to_async(thread_sensitive=True)

susmithagudapati

class User(models.Model):
    name = models.CharField(max_length=64, null=True)

class Category(models.Model):
    name = models.CharField(max_length=128)
    is_active = models.BoolenField(default=False)
    created_by = models.ForeignKey(
        User, on_delete=models.PROTECT, related_name='category_created_by', null=True)
    
class ProductAttribute(models.Model):

susmithagudapati

# Hits the database.
product = Product.objects.get(id=10)

# Hits the database again to get the related Category object.
category = product.category


# using select_related
# Hits the database
product = Product.objects.select_related('category').get(id=10)

susmithagudapati

>>> Product.objects.filter(is_active=True).count()
200

category_names = set()

# Hits the database 201 times. 1 for fetching active products 
# and the rest 200 queries are for retrieving category for each product.
products = Product.objects.filter(is_active=True)
for product in products:
  category_names.add(product.category.name)

susmithagudapati

product = Product.objects.get(id=10) # Hits the database.

category = product.category # Hits the database again.
user = category.created_by # Hits the database again!


# using select_related
product = Product.objects.select_related('category__created_by').get(id=10) # Hits the database

category = product.category # Doesn't hit the database

susmithagudapati

# create indexes using Field.db_index value
class Product(models.Model):
    name = models.CharField(max_length=128, db_index=True)
    upc = models.CharField(max_length=32, unique=True)
    category = models.ForeignKey(
        Category, on_delete=models.PROTECT, related_name='categories', null=True)
    created_by = models.ForeignKey(
        User, on_delete=models.PROTECT, related_name='product_created_by', null=True)
    
    

susmithagudapati

>>> Product.objects.count()
220

# 220 queries - __str__ is called for each product
Product.objects.all()
[ "Jeans (cotton, skinny)", "Neon Shirt (attrx, attry)", ...]


# 2 queries - 1 query for products and 1 query for attributes.
# attributes are prefetched when products query is executed.