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Difference between select_related and prefetch_related in Django?

By Editorial @stacktips, On Sep 17, 2023 Django 2.84K Views

Accessing related objects through the ORM can sometimes lead to performance bottlenecks, especially when dealing with large datasets or complex relationships. One common issue is the "N+1 queries" problem, where a query is executed for each related object, leading to a significant increase in database queries and slow performance.

To avoid this problem in Django, we can use select_related and prefetch_related are methods that used to optimize the database queries when dealing with related objects.

Using select_related method

The select_related is used to retrieve related objects using a single SQL query. It works by following foreign key relationships and joining the related objects into a single query. This can be useful when you need to access related objects in a loop or when accessing a single related object in a template.

For example, consider the following code:

# Retrieve all books and their authors using select_related
books = Book.objects.select_related('author')
for book in books:
    print(book.title, book.author.name)

In this example, select_related is used to retrieve all books and their authors in a single query.

Using prefetch_related method

On the other hand, prefetch_related is used to retrieve related objects using multiple queries. It works by first retrieving the main queryset, and then retrieving the related objects in a separate query. This can be useful when you need to access many related objects, or when you want to avoid unnecessary database joins.

For example, consider the following code:

# Retrieve all books and their reviews using prefetch_related
books = Book.objects.prefetch_related('reviews')
for book in books:
    print(book.title)
    for review in book.reviews.all():
        print(review.body)

In this example, prefetch_related is used to retrieve all books and their reviews in two separate queries.

In general, you should use select_related when accessing a single related object, or when accessing related objects in a loop.

Use prefetch_related when accessing many related objects or when you want to avoid unnecessary database joins.

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