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Django QuerySet Optimization with select_related vs prefetch_related

# Prasath's Django application has these models:
class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

class Review(models.Model):
    book = models.ForeignKey(Book, on_delete=models.CASCADE)
    rating = models.IntegerField()

# Prasath needs to fetch all reviews with their related books and authors
reviews = Review.objects.select_related('book__author').all()

  1. The query is optimal because select_related() handles both forward ForeignKey relationships and will fetch Author data in a single query with JOINs.

  2. The query should use prefetch_related('book__author') instead to avoid database table JOINs and use separate queries.

  3. The query should combine both: select_related('book').prefetch_related('book__author') for better performance.

  4. The query is inefficient because select_related() creates unnecessary JOINs and should use only filter() instead.

Show answer & explanation

Correct answer

The query is optimal because select_related() handles both forward ForeignKey relationships and will fetch Author data in a single query with JOINs.

Explanation

select_related() performs SQL JOINs for ForeignKey relationships (one-to-one and one-to-many from the one side), while prefetch_related() uses separate queries for reverse ForeignKey and ManyToMany. For the given code, select_related('book__author') correctly follows the ForeignKey chain.

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All 30 Django Models and Database Design questions

  1. 1.What is the primary purpose of Django Models?
  2. 2.Which field type should be used to store an email address in a Django Model?
  3. 3.What does the Meta class inside a Django Model do?
  4. 4.Which relationship type is used when one model can have many instances of another model?
  5. 5.What is the purpose of the `__str__` method in a Django Model?
  6. 6.Which field attribute makes a field mandatory in Django Models?
  7. 7.What does the `on_delete` parameter do in a ForeignKey relationship?
  8. 8.Which of the following is a valid Django field type for storing boolean values?
  9. 9.What is the default behavior when you don't specify `null=True` for a field in Django?
  10. 10.Which method is used to retrieve a single object from the database in Django?
  11. 11.Understanding Django Model Meta Options
  12. 12.Scenario:
  13. 13.Django QuerySet Filter vs Get Method
  14. 14.Coding:
  15. 15.Scenario:
  16. 16.Understanding Django Model Inheritance
  17. 17.Coding:
  18. 18.Scenario:
  19. 19.Django Model Manager and QuerySet Methods
  20. 20.Coding:
  21. 21.Django Model Inheritance Strategy Selection
  22. 22.Django QuerySet Optimization with select_related vs prefetch_related
  23. 23.Django Model Field Choices and Validation
  24. 24.Django Model Relationships:
  25. 25.Django Model Signals and Database Consistency
  26. 26.Django Model Meta Options:
  27. 27.Django Model Managers and Custom QuerySets
  28. 28.Django Model Field:
  29. 29.Django Model Validation:
  30. 30.Django Model Aggregation and Annotation