QuerySet

Django · Reference cheat sheet

📋 Overview

A QuerySet is a lazy, chainable representation of a DB query. Evaluation happens on iteration, list(), bool(), slicing with step, or terminal methods (get, count, exists, aggregate). Optimize with select_related / prefetch_related and defer unused fields.

🔧 Core concepts

Lookups: field__lookup (exact, iexact, contains, in, gt, isnull, …).

💡 Examples

Filtering and Q objects:

from django.db.models import Q, Count

Article.objects.filter(is_published=True).exclude(author=None)

Article.objects.filter(
    Q(title__icontains="django") | Q(tags__name="python")
).distinct()

Related + annotate:

qs = (
    Article.objects.filter(is_published=True)
    .select_related("author")
    .prefetch_related("tags")
    .annotate(tag_count=Count("tags"))
    .order_by("-published_at")
)

Updates and existence:

Article.objects.filter(pk=1).update(is_published=True)  # no signals
Article.objects.filter(slug="hello").exists()
Article.objects.values_list("id", flat=True)

⚠️ Pitfalls

• N+1: accessing .author / .tags.all() in a loop without prefetch.
• Evaluating the same queryset twice unexpectedly (cache after first eval).
• get() raises DoesNotExist / MultipleObjectsReturned.
• Chaining after slicing can be restricted; prefer filter before slice.
• update() / bulk_create skip save() and most signals.

🔗 Related

• Models
• Managers
• Transactions / ORM
• Pagination
• Testing