Prefetch_related for reverse relations in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does prefetch_related do in Django ORM?

prefetch_related helps fetch related objects in a separate query and joins them in Python. It reduces database hits when accessing related objects, especially for reverse or many-to-many relations.

beginner

How do you use prefetch_related for reverse relations in Django?

Use the name of the related manager or related_name on the reverse side inside prefetch_related. For example, if Book has a foreign key to Author, use Author.objects.prefetch_related('book_set') or the custom related_name.

intermediate

Why is prefetch_related better than select_related for reverse relations?

select_related works only for single-valued forward relations (foreign keys). Reverse relations can have many objects, so prefetch_related fetches them in a separate query and joins in Python, avoiding inefficient joins.

intermediate

What is the benefit of using Prefetch object with prefetch_related?

The Prefetch object lets you customize the queryset used for prefetching, like filtering or ordering related objects before joining them. This gives more control over the data fetched for reverse relations.

beginner

Example: How to prefetch all Book objects for each Author using reverse relation?

authors = Author.objects.prefetch_related('book_set')
for author in authors:
    books = author.book_set.all()  # No extra query here
    print(author.name, books)

Which method fetches related objects in a separate query and joins them in Python?

A annotate

B select_related

C prefetch_related

D filter

prefetch_related fetches related objects in a separate query and joins them in Python, ideal for reverse or many-to-many relations.

For a reverse foreign key relation, which argument is correct to use with prefetch_related?

A The related manager name or related_name

B The foreign key field name

C The model class name

D The database table name

You use the related manager name or the related_name defined on the foreign key for reverse relations.

Why should you not use select_related for reverse relations?

A It causes syntax errors

B It only works for single-valued forward relations

C It fetches data in multiple queries

D It does not support filtering

select_related only works for single-valued forward relations like foreign keys, not reverse or many-to-many relations.

What does the Prefetch object allow you to do?

A Customize the queryset used for prefetching related objects

B Change the database connection

C Modify model fields dynamically

D Create new database tables

Prefetch lets you customize the queryset for prefetching, such as filtering or ordering related objects.

What happens when you access a reverse relation after using prefetch_related?

A An error is raised

B A new query is executed each time

C The data is not available

D No extra database query is made

After prefetching, accessing the reverse relation uses cached data without extra queries.

Explain how prefetch_related improves performance when accessing reverse relations in Django.

Think about how Django avoids querying the database repeatedly.

Describe how to use the Prefetch object with prefetch_related to filter reverse related objects.

Consider customizing the related objects you want to fetch.