What is the querySet() method in Django?

QuerySet in Django represents a collection of objects retrieved from the database using a model manager. It allows filtering, ordering, and manipulating data before retrieval. QuerySet is "lazy," meaning database queries are executed only when the data is accessed (e.g., during iteration or when calling methods like list(), count()).

Main features and capabilities:

• Lazy execution: Queries are executed only when necessary.
• Chaining: QuerySet methods return a new QuerySet, allowing sequential application of filters and transformations.
• Caching: After the first data access, QuerySet caches the data in memory for quick subsequent retrieval.

Examples of usage:

# managers.py
from django.db import models

class MyModelManager(models.Manager):
    def published(self):
        return self.get_queryset().filter(is_published=True)

# models.py
from django.db import models
from .managers import MyModelManager

class Article(models.Model):
    title = models.CharField(max_length=200)
    body = models.TextField()
    is_published = models.BooleanField(default=False)

    objects = MyModelManager()

# views.py
from django.shortcuts import render
from .models import Article

def published_articles(request):
    articles = Article.objects.published() # Using custom QuerySet
    return render(request, 'articles/published.html', {'articles': articles})

Main methods of QuerySet:

Method	Description
filter(**kwargs)	Returns a new QuerySet containing objects matching the conditions.
exclude(**kwargs)	Returns a new QuerySet containing objects not matching the conditions.
get(**kwargs)	Returns a single object matching the conditions or raises an exception.
all()	Returns all objects in the QuerySet.
order_by(*fields)	Returns a new QuerySet ordered by specified fields.
count()	Returns the number of objects in the QuerySet.
exists()	Returns True if the QuerySet contains at least one object.
values(*fields)	Returns a QuerySet of dictionaries instead of model instances.
values_list(*fields)	Returns a QuerySet of tuples instead of model instances.

Custom managers (inheriting from models.Manager) allow adding their own methods for working with QuerySet, making the code more declarative and reusable.