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27 / 40Django ORM Queries and QuerySetsHardPython - Django Framework

Aggregation and Annotation in Django ORM

from django.db.models import Count, Avg, Q
from myapp.models import Student, Course

# Query
results = Student.objects.annotate(
    total_courses=Count('course'),
    avg_rating=Avg('course__rating')
).filter(
    total_courses__gte=3,
    avg_rating__gt=4.0
).values('name', 'total_courses', 'avg_rating')

  1. The query returns students with 3+ courses and average course rating > 4.0, including the calculated total_courses and avg_rating fields.

  2. The query will raise an error because annotate() cannot be used with filter() on annotated fields.

  3. The query returns all students with their course counts, then filters to show only those with avg_rating > 4.0.

  4. The annotate() method returns a dictionary queryset without including the annotated fields in the output.

Show answer & explanation

Correct answer

The query returns students with 3+ courses and average course rating > 4.0, including the calculated total_courses and avg_rating fields.

Explanation

annotate() adds calculated fields to each object in the QuerySet. These annotations can be used in subsequent filter() calls. The query returns students enrolled in 3+ courses with average rating > 4.0, including the calculated fields.

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All 40 Django ORM Queries and QuerySets questions

  1. 1.What is a QuerySet in Django ORM?
  2. 2.Which method is used to retrieve all objects from a model?
  3. 3.What does the `filter()` method return in Django ORM?
  4. 4.What is the difference between `filter()` and `get()` methods?
  5. 5.What does the `exclude()` method do in Django ORM?
  6. 6.How do you order QuerySet results in Django?
  7. 7.What does `count()` method return?
  8. 8.What is QuerySet slicing used for?
  9. 9.What does `exists()` method do?
  10. 10.What does the `distinct()` method do?
  11. 11.Understanding Django QuerySet Evaluation
  12. 12.Filtering Related Objects with select_related()
  13. 13.Scenario:
  14. 14.Understanding F() Objects in Django ORM
  15. 15.Aggregation with Django ORM
  16. 16.Scenario:
  17. 17.Understanding Prefetch_related() for Reverse Relations
  18. 18.Query Chaining and Method Order
  19. 19.Using values() vs values_list()
  20. 20.Scenario:
  21. 21.Understanding QuerySet Lazy Evaluation and Database Queries
  22. 22.Scenario:
  23. 23.Analyzing N+1 Query Problem in Django ORM
  24. 24.QuerySet Method Chaining and Filter Behavior
  25. 25.Scenario:
  26. 26.Understanding QuerySet Slicing and Caching Behavior
  27. 27.Aggregation and Annotation in Django ORM
  28. 28.Scenario:
  29. 29.Understanding F Objects and Database-Level Operations
  30. 30.Scenario:
  31. 31.Complex Query with Multiple Aggregations and Filtering
  32. 32.Scenario:
  33. 33.Understanding Subqueries and Exists()
  34. 34.Raw SQL Queries and Security
  35. 35.Case-Insensitive Lookups and Performance
  36. 36.Scenario:
  37. 37.Understanding Query Caching and Database Hits
  38. 38.Bulk Operations:
  39. 39.Scenario:
  40. 40.Understanding Query Optimization with explain()