Django Interview Preparation

 Django's ORM makes database interactions seamless, allowing developers to write queries in Python without raw SQL. However, as applications scale, inefficient queries can slow down performance, leading to high latency and database load.  This guide explores advanced query optimization techniques in Django ORM to go beyond basic CRUD (Create, Read, Update, Delete) operations and improve efficiency.  1. Use QuerySet Caching to Avoid Repeated Queries Using cache reduces redundant queries for frequently accessed data. Caching helps reduce repeated database hits. 2. Avoid .count() on Large Datasets Using .count() on large tables can be expensive Inefficient way: Optimized way ( .exists() is Faster) 3. Use Indexes for Faster Lookups Indexes speed up queries on frequently filtered fields. Add db_index=True for frequently queried fields: 4. Optimize Bulk Inserts and Updated Performing operations on multiple records one by one is inefficient. Use bulk_create() for mass insert...

Django Request Life Cycle  A web application or a website revolves around the request-response cycle and Django applications are no exception to this. But it is not just a two step process. Our Django applications needs to go through various stages to return the end user some result. To understand the Django framework better we must understand how the requests are initiated and the end result is served to the end user. When setting up a new Django project, one of the first things you’ll do is wire up your URLconfs and set up some views. But what’s actually happening under the hood here? How does Django route traffic to the view, and what part do middlewares play in this cycle? Layers of Django Application Request Middlewares URL Router(URL Dispatcher) Views Context Processors Template Renderers Response Middlewares Whenever a request comes in first it goes to the web server (Ngnix /Apache) . The the request goes to django's WSGI (Web Server Gateway Interface) / ASGI  (Asynchr...

When optimizing a Django project for large write operations, especially when dealing with PostgreSQL, there are several strategies you can employ to reduce the time it takes to perform these operations: 1. Bulk Inserts In django, we create objects using create()  . Asynchronous version is acreate() .It's a  convenience method for creating an object and saving it all in one step.  and  These are same and equivalent. The create() method is used to create and save a single object in the database. Example: Instead of inserting one row at a time, consider using Django's bulk_create() method to insert multiple rows in a single query. This reduces the overhead of multiple database round trips. Example:  The bulk_create() method is used to create and save multiple objects in the database in a single query. It accepts a list of model instances and inserts them into the database in a single batch operation, which significantly reduces the overhead compared to individ...

  Django Password Django provides a flexible password storage system and uses PBKDF2 by default. Django saves the password as below. <algorithm>$<iterations>$<salt>$<hash> example of a Hashed password stored in database: pbkdf2_sha256$390000$LCm33kvO7rbjbZhwJA90Sf$xfuGOzl/MJyUxqWNhsNdSThaQUvn1EjEfxZ48HA8HF4= Those are the components used for storing a User’s password,separated by the dollar-sign character and consist of:  1. The hashing algorithm 2. The number of algorithm iterations (work factor) 3. The random salt 4. The resulting password hash.  Most password hashes include a salt along with their password hash in order to protect against rainbow table attacks. Example of Making Hashed password: Here’s a simplified overview of how Django handles password storage: 1. Password Creation or Change : # When someone creates a new account or decides to change their password, Django takes their chosen password and performs a process called hashing. Has...

  Database Indexing Database indexing is a technique used to optimize the performance of database queries by allowing the database management system (DBMS) to quickly locate and retrieve specific rows of data. Indexes are data structures that provide a faster way to look up records based on the values stored in one or more columns of a table. When you create an index on a table, the DBMS creates a separate data structure that maps the values in the indexed columns to the corresponding rows in the table. Default Type of Index is B-Tree Index ( The king of all indexes) বইতে কোন টপিক খুজতে গেলে আমরা টেবিল অফ কনটেন্ট থেকে দেখি এই টপিক কত নম্বর পেজে আছে।যাতে করে আমাদের পুরো বই খুজতে না হয়। ডেটাবেজ ইনডেক্সিং ও তেমনই একটা ইফিসিয়েন্ট টেকনিক।ডেটাবেজে কোন ডেটাকে দ্রুত খুজে বের করার জন্য ইনডেক্সিং করা লাগে।যদি এমন হয় একটা কুয়েরি বার বার এক্সিকিউট করতে হচ্ছে এবং একটা কলাম থেকে ভ্যালু বার বার খুজতে হচ্ছে তখন আমরা সেই কলামে ইনডেক্সিং করতে পারি।এর মাধ্যমে কোন ডেটা দ্রুত রিট্রাইভ করা যায়।কিন্তু ই...

  Difference between select_related and prefetch_related Reducing SQL queries is one of the first steps when optimizing a Django project. There are two powerful methods included in the Django ORM to help us that can boost performance by creating a single more complex QuerySet rather than multiple, smaller queries. In this project we will understand about  select_related and prefetch_related.  Django use these two orm method to reduce sql queries in database based on different scenario.  select_related Lets assume  this two model we have.  class Author ( models . Model ): name = models . CharField ( max_length = 200 ) def __str__ ( self ): return self . name class Courses ( models . Model ): name = models . CharField ( max_length = 200 ) author = models . ForeignKey ( Author , on_delete = models . CASCADE , related_name = 'courses' ) def __str__ ( self ): return self . name Here we have two mode. ...

 Django Slugs and UUIDs Why We have to Use UUID or SlugFlied: The django default id or primary_key( pk ) is currently  auto-incrementing id. Among other concerns, it tells a potential hacker exactly how many records you have in your database; it tells them exactly what the id is which can be used in a potential attack; and there can be synchronization issues if you have multiple front-ends.  There are two alternative approaches. UUID and SlugFiled UUIDField: A field for storing universally unique identifiers. Uses Python’s UUID class. When used on PostgreSQL and MariaDB 10.7+, this stores in a uuid datatype, otherwise in a char(32). Example Code: class UserProfile ( models . Model ): id = models . UUIDField ( primary_key = True , default = uuid . uuid4 , editable = False ) name = models . CharField ( max_length = 500 ) email = models . EmailField () def __str__ ( self ): return self . name def get_absolute_url ( self ): ...

 Django pk VS id If you don’t specify primary_key=True for any fields in your model, Django will automatically add an IntegerField to hold the primary key, so you don’t need to set primary_key=True on any of your fields unless you want to override the default primary-key behavior. The primary key field is read-only. If you change the value of the primary key on an existing object and then save it, a new object will be created alongside the old one Example: class UserProfile ( models . Model ): name = models . CharField ( max_length = 500 ) email = models . EmailField ( primary_key = True ) def __str__ ( self ): return self . name suppose we have this model. In this model we have make email field as primary key. now django default primary key id field will be gone. It'll remove from database. we can not query as   UserProfile.objects.get(id=1) after make email as primary key this query will throw an error.  Now we have to use pk  Us...