Prefetch_related for reverse relations in Django - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to prefetch the reverse relation 'books' for all authors.

Django

authors = Author.objects.all().[1]('books')

Drag options to blanks, or click blank then click option'

Afilter

Bprefetch_related

Cselect_related

Dannotate

Attempts:

3 left

💡 Hint

Common Mistakes

Using select_related instead of prefetch_related for reverse relations.

Forgetting to specify the related name 'books'.

Use prefetch_related to efficiently fetch reverse relations like 'books' for authors.

2fill in blank

medium

Complete the code to prefetch the reverse relation 'comments' on a queryset of posts.

Django

posts = Post.objects.all().[1]('comments')

Drag options to blanks, or click blank then click option'

Aorder_by

Bannotate

Cprefetch_related

Dvalues

Attempts:

3 left

💡 Hint

Common Mistakes

Using annotate which is for aggregation, not fetching related objects.

Using order_by which only sorts the queryset.

prefetch_related is used to fetch reverse relations like 'comments' efficiently.

3fill in blank

hard

Fix the error in the code to correctly prefetch the reverse relation 'orders' for customers.

Django

customers = Customer.objects.all().[1]('order_set')

Drag options to blanks, or click blank then click option'

Aselect_related

Bexclude

Cfilter

Dprefetch_related

Attempts:

3 left

💡 Hint

Common Mistakes

Using select_related which only works for forward foreign keys.

Using incorrect related name instead of 'order_set'.

Use prefetch_related with the default reverse relation name 'order_set' to fetch orders for customers.

4fill in blank

hard

Fill both blanks to prefetch the reverse relation 'reviews' and filter only those with rating greater than 3.

Django

products = Product.objects.all().prefetch_related([1]('reviews', queryset=Review.objects.filter(rating [2] 3)))

Drag options to blanks, or click blank then click option'

APrefetch

B>

C<

Dselect_related

Attempts:

3 left

💡 Hint

Common Mistakes

Using select_related instead of Prefetch for custom queryset.

Using wrong comparison operator in filter.

Use Prefetch to customize the prefetch with a filtered queryset, and use > to filter ratings greater than 3.

5fill in blank

hard

Fill all three blanks to prefetch reverse relation 'entries' with a filtered queryset, and order the entries by 'date'.

Django

blogs = Blog.objects.all().prefetch_related([1]('entries', queryset=Entry.objects.filter(status=[2]).order_by([3])))

Drag options to blanks, or click blank then click option'

APrefetch

B'published'

C'date'

Dselect_related

Attempts:

3 left

💡 Hint

Common Mistakes

Using select_related instead of Prefetch.

Forgetting to quote string values in filter and order_by.

Use Prefetch to customize the prefetch, filter entries with status 'published', and order by 'date'.

Practice

1. What is the main purpose of using prefetch_related with reverse relations in Django?

easy

A. To update related objects in bulk

B. To fetch related objects in a single query and reduce database hits

C. To delete related objects automatically

D. To create new related objects during query

Solution

1. Step 1: Understand what prefetch_related does

   prefetch_related fetches related objects in a separate query but combines results in Python to reduce database hits.

2. Step 2: Apply this to reverse relations

   Using prefetch_related on reverse relations loads all related objects efficiently, avoiding queries inside loops.

3. Final Answer:

   To fetch related objects in a single query and reduce database hits -> Option B

4. Quick Check:

   prefetch_related reduces queries [OK]

Hint: Remember: prefetch_related loads reverse relations in one go [OK]

Common Mistakes:

• Thinking prefetch_related deletes or updates data
• Confusing prefetch_related with select_related
• Assuming it creates new objects automatically

2. Which of the following is the correct syntax to prefetch a reverse relation named comments on a Post model?

easy

A. Post.objects.prefetch_related(comments)

B. Post.objects.select_related('comments')

C. Post.objects.prefetch('comments')

D. Post.objects.prefetch_related('comments')

Solution

1. Step 1: Recall the correct method name

   The correct method to prefetch related objects is prefetch_related, not prefetch or select_related for reverse relations.

2. Step 2: Check the argument format

   The related name must be a string inside quotes, so 'comments' is correct, not a variable without quotes.

3. Final Answer:

   Post.objects.prefetch_related('comments') -> Option D

4. Quick Check:

   Correct method and string argument [OK]

Hint: Use prefetch_related('relation_name') with quotes [OK]

Common Mistakes:

• Using select_related for reverse relations
• Omitting quotes around relation name
• Using a non-existent method like prefetch()

3. Given these models:

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE)
    title = models.CharField(max_length=100)

What will this code print?

authors = Author.objects.prefetch_related('books')
for author in authors:
    print(author.name, [book.title for book in author.books.all()])

medium

A. Prints author names but book lists are empty

B. Raises an error because 'books' is not a valid relation

C. Prints each author name with a list of their book titles without extra queries

D. Prints author names but causes a query for each author inside the loop

Solution

1. Step 1: Understand the model relation

   Each Author has many Book objects accessible via the reverse relation books.

2. Step 2: Analyze the prefetch_related effect

   Using prefetch_related('books') loads all books for all authors in one extra query, so no queries happen inside the loop.

3. Final Answer:

   Prints each author name with a list of their book titles without extra queries -> Option C

4. Quick Check:

   prefetch_related loads reverse relations efficiently [OK]

Hint: prefetch_related loads reverse sets before looping [OK]

Common Mistakes:

• Thinking prefetch_related causes errors if relation exists
• Assuming empty lists because of missing select_related
• Believing queries run inside the loop despite prefetch

4. What is wrong with this code snippet?

posts = Post.objects.prefetch_related('comment_set')
for post in posts:
    print(post.title, [c.text for c in post.comment_set.all()])

Assuming Comment model has a ForeignKey to Post without a related_name set.

medium

A. Using 'comment_set' is correct; no error here

B. Should use 'comments' instead of 'comment_set' in prefetch_related

C. Must use select_related instead of prefetch_related for reverse relations

D. The code will raise an AttributeError because 'comment_set' is invalid

Solution

1. Step 1: Understand default reverse relation naming

   If no related_name is set on a ForeignKey, Django uses modelname_set as the reverse relation name, here comment_set.

2. Step 2: Check usage in prefetch_related and loop

   Using prefetch_related('comment_set') and accessing post.comment_set.all() is correct and will work without error.

3. Final Answer:

   Using 'comment_set' is correct; no error here -> Option A

4. Quick Check:

   Default reverse name = modelname_set [OK]

Hint: Default reverse name is modelname_set if no related_name [OK]

Common Mistakes:

• Assuming related_name is always 'comments'
• Using select_related for reverse relations
• Expecting an error when using default reverse name

5. You want to efficiently load all Author objects with their Book objects, but only books published after 2020. How do you use prefetch_related for this filtered reverse relation?

authors = Author.objects.prefetch_related( ... )

hard

A. Use Prefetch with a filtered queryset: Prefetch('books', queryset=Book.objects.filter(pub_year__gt=2020))

B. Use prefetch_related('books').filter(pub_year__gt=2020)

C. Use select_related('books').filter(pub_year__gt=2020)

D. Use prefetch_related('books__filter(pub_year__gt=2020)')

Solution

1. Step 1: Understand filtering reverse relations with prefetch_related

   To filter related objects in prefetch_related, use the Prefetch object with a filtered queryset.

2. Step 2: Apply Prefetch with filtered queryset

   Use Prefetch('books', queryset=Book.objects.filter(pub_year__gt=2020)) inside prefetch_related() to load only books after 2020.

3. Final Answer:

   Use Prefetch with a filtered queryset: Prefetch('books', queryset=Book.objects.filter(pub_year__gt=2020)) -> Option A

4. Quick Check:

   Filtered prefetch needs Prefetch object [OK]

Hint: Use Prefetch with filtered queryset inside prefetch_related [OK]

Common Mistakes:

• Trying to filter directly inside prefetch_related string
• Using select_related for many-to-one reverse relations
• Filtering the main queryset instead of related queryset