Aggregation with Count and Sum
Counting Rows
The simplest way to count matching rows is QuerySet.count(), which asks the database for a number instead of loading every row into Python.
Note: count() is faster than len(queryset) because it doesn't fetch the actual rows.
Example: Counting Rows
This returns a single integer — the number of published tutorial rows — without loading their data.
from core.models import Tutorial
# How many tutorials are published?
Tutorial.objects.filter(is_published=True).count()Using aggregate() with Count and Sum
aggregate() runs one or more aggregation functions across a whole queryset and returns a dictionary of results.
Example: Using aggregate() with Count and Sum
This returns something like {total: 15, total_order: 120} — one dictionary summarizing the whole queryset.
from django.db.models import Count, Sum
from core.models import Chapter
Chapter.objects.aggregate(
total=Count('id'),
total_order=Sum('order'),
)Counting Related Objects
Count can also count how many related objects each row has, which is useful for showing 'how many tutorials does this chapter have'.
Warning: Count() on a related field only works when a ForeignKey links the two models.
Example: Counting Related Objects
Each Chapter object in the result now has a .tutorial_count attribute showing how many related Tutorial rows point to it.
from django.db.models import Count
from core.models import Chapter
Chapter.objects.annotate(tutorial_count=Count('tutorials'))- Confusing .count() (a QuerySet method that returns one number) with Count (an aggregation function used inside .aggregate() or .annotate()).
- Forgetting to import Count/Sum from django.db.models before using them.
- Using aggregate() when annotate() was needed (or vice versa) — aggregate() returns one summary for the whole queryset, annotate() adds a value to each object.
- aggregate() computes a single summary value across an entire queryset.
- Count and Sum are imported from django.db.models and used inside aggregate().
- annotate() adds a computed value to each object in a queryset instead of one overall summary.
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