Prefetch_related for reverse relations in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does prefetch_related do with reverse relations?

Consider two Django models: Author and Book, where Book has a foreign key to Author. If you use Author.objects.prefetch_related('book_set'), what is the main effect?

A It does not affect the query and fetches authors without related books.

B It joins the author and book tables in one query using SQL JOIN.

C It fetches only the first book for each author to optimize performance.

D It fetches all related books for each author in a single additional query, reducing database hits.

💡 Hint

Think about how Django avoids querying the database multiple times for related objects.

prefetch_related fetches related objects in a separate query and joins them in Python, which is efficient for reverse relations like book_set.

❓ state_output

intermediate

How many queries are executed with prefetch_related on reverse relation?

Given the models Author and Book (with Book.author as ForeignKey), what is the number of database queries executed by this code?

authors = Author.objects.prefetch_related('book_set').all()
for author in authors:
    print(len(author.book_set.all()))

A No queries: data is fetched lazily when printing.

B 1 query: a single JOIN query fetching authors and books.

C 2 queries: one for authors and one for all related books.

D N+1 queries: one for authors and one per author for books.

💡 Hint

Remember how prefetch_related works internally with separate queries.

prefetch_related runs one query for the main model and one for the related model, then combines results in Python, so total 2 queries.

🔧 Debug

advanced

Why does prefetch_related('books') raise an error on reverse relation?

Given models Author and Book where Book.author is a ForeignKey, what is wrong with this query?

Author.objects.prefetch_related('books').all()

Assuming no related_name is set on Book.author.

A prefetch_related cannot be used on reverse relations without related_name.

B The related name 'books' does not exist; the default reverse relation is 'book_set'.

C The query will run but return empty related objects.

D The query causes a syntax error due to missing parentheses.

💡 Hint

Check the default reverse relation name Django uses when no related_name is set.

Without a related_name, Django uses modelname_set as the reverse relation name, so books is invalid.

📝 Syntax

advanced

Which prefetch_related syntax correctly fetches nested reverse relations?

Given models Author, Book, and Chapter where Book has ForeignKey to Author and Chapter has ForeignKey to Book, which syntax correctly prefetches all books and their chapters for authors?

A Author.objects.prefetch_related('book_set__chapter_set').all()

B Author.objects.prefetch_related('book_set.chapter_set').all()

C Author.objects.prefetch_related(['book_set', 'chapter_set']).all()

D Author.objects.prefetch_related('book_set').prefetch_related('chapter_set').all()

💡 Hint

Think about how to specify nested relations in prefetch_related.

Use double underscores to chain reverse relations in prefetch_related. Option A is correct syntax.

🧠 Conceptual

expert

Why use Prefetch object with custom queryset on reverse relation?

When prefetching a reverse relation in Django, why might you use Prefetch with a custom queryset instead of a simple string?

A To filter or order the related objects before they are attached to the main objects.

B To force Django to use SQL JOIN instead of separate queries.

C To disable caching of related objects for memory optimization.

D To automatically update related objects in the database.

💡 Hint

Consider why you might want to control which related objects are fetched.

The Prefetch object lets you specify a custom queryset to filter, order, or annotate related objects before they are attached, improving query control.