Relationship query patterns in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Django ORM query?

Given models Author and Book where Book has a ForeignKey to Author, what does this query return?

Author.objects.filter(book__title__icontains='django').distinct()

A A queryset of authors who have no books with 'django' in the title

B A queryset of books with 'django' in the title

C A queryset of all authors regardless of their books

D A queryset of authors who have at least one book with 'django' in the title, without duplicates

💡 Hint

Think about how the double underscore syntax works in Django ORM for related fields.

The filter book__title__icontains='django' looks for authors whose related books have titles containing 'django'. The distinct() removes duplicates if an author has multiple matching books.

❓ state_output

intermediate

How many items are in the resulting queryset?

Consider these models:
class Publisher(models.Model): name = models.CharField(max_length=100)
class Book(models.Model): title = models.CharField(max_length=100); publisher = models.ForeignKey(Publisher, on_delete=models.CASCADE)

Given 3 publishers and 5 books distributed among them, what is the count of this queryset?

Publisher.objects.filter(book__title__startswith='A').distinct().count()

A 5

B 3

C 2

D 0

💡 Hint

Count publishers who have at least one book starting with 'A'.

The filter selects publishers with books whose titles start with 'A'. If 2 publishers have such books, the count is 2.

📝 Syntax

advanced

Which option correctly uses Django ORM to get all books with authors named 'Alice'?

Given Book with ForeignKey author to Author, which query is correct?

A Book.objects.filter(author__name='Alice')

B Book.objects.filter(author.name='Alice')

C Book.objects.filter('author__name'='Alice')

D Book.objects.filter(author.name=='Alice')

💡 Hint

Use double underscores to traverse relationships in filters.

Django ORM uses double underscores to access related model fields in filters. Option A is correct syntax.

🔧 Debug

advanced

What error does this Django ORM query raise?

Given models Category and Product where Product has ForeignKey category, what error occurs with this query?

Product.objects.filter(category__name__in='Electronics,Books')

A TypeError: argument of type 'str' is not iterable

B SyntaxError: invalid syntax

C No error, returns products in categories 'Electronics' or 'Books'

D ValueError: invalid lookup 'in' for CharField

💡 Hint

Check the type expected by the __in lookup.

The __in lookup expects an iterable like a list or tuple, but a string was given, causing a TypeError.

🧠 Conceptual

expert

Which option best describes the behavior of select_related in Django ORM?

What does select_related do when querying related models?

A It fetches related objects lazily when accessed, causing additional queries

B It performs a SQL join and fetches related objects in the same query to reduce database hits

C It caches all related objects in memory for the entire application runtime

D It deletes related objects automatically when the main object is deleted

💡 Hint

Think about how to optimize queries to avoid multiple database hits.

select_related uses SQL joins to get related objects in one query, improving performance by reducing queries.