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30 / 30Django Models and Database DesignHardPython - Django Framework

Django Model Aggregation and Annotation

from django.db.models import Count, Avg, Q

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)
    rating = models.IntegerField()
    pages = models.IntegerField()

# Prasath wants to find authors with more than 5 books and average rating > 4:
result = Author.objects.annotate(
    book_count=Count('book'),
    avg_rating=Avg('book__rating')
).filter(book_count__gt=5, avg_rating__gt=4)

# Prasath also wants to get the total pages across all books:
total = Book.objects.aggregate(total_pages=Sum('pages'))

  1. The code will fail because aggregate() cannot be used after annotation in the same queryset.

  2. The code is correct: annotate() adds book_count and avg_rating to each Author object, filter() narrows results, and aggregate() computes total_pages across all books.

  3. The aggregate() call should use annotate() instead to calculate total pages per author.

  4. The filter() condition will not work because avg_rating is not a database field.

Show answer & explanation

Correct answer

The code is correct: annotate() adds book_count and avg_rating to each Author object, filter() narrows results, and aggregate() computes total_pages across all books.

Explanation

Annotation adds calculated fields to each object in the queryset, while aggregation returns a single dictionary with aggregate values across the entire queryset. The query correctly uses annotate() for per-author calculations and aggregate() for global totals.

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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