aggregate() vs annotate() in Django

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I would focus on the example queries rather than your quote from the documentation. Aggregate calculates values for the entire queryset. Annotate calculates summary values for each item in the queryset.

Aggregation

>>> Book.objects.aggregate(average_price=Avg('price'))
{'average_price': 34.35}

Returns a dictionary containing the average price of all books in the queryset.

Annotation

>>> q = Book.objects.annotate(num_authors=Count('authors'))
>>> q[0].num_authors
2
>>> q[1].num_authors
1

q is the queryset of books, but each book has been annotated with the number of authors.

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