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21 / 40Django ORM Queries and QuerySetsHardPython - Django Framework

Understanding QuerySet Lazy Evaluation and Database Queries

from django.db import connection
from django.test.utils import CaptureQueriesContext
from myapp.models import Student

with CaptureQueriesContext(connection) as queries:
    qs = Student.objects.filter(age__gte=18)
    print(f"Queries after filter: {len(queries)}")
    
    qs = qs.filter(city='Mumbai')
    print(f"Queries after second filter: {len(queries)}")
    
    students = list(qs)
    print(f"Queries after list(): {len(queries)}")

  1. The output will show 0 queries after both filters, and 1 query after list().

  2. The output will show 1 query after the first filter, 2 queries after the second filter, and 3 queries after list().

  3. The output will show 1 query after each filter() call, and 3 queries total after list().

  4. The output will show an error because CaptureQueriesContext cannot be used with QuerySet filters.

Show answer & explanation

Correct answer

The output will show 0 queries after both filters, and 1 query after list().

Explanation

QuerySets are lazy-evaluated; filters don't execute queries until the queryset is evaluated (converted to list, iterated, or sliced).

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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()