How to execute a GROUP BY ... COUNT or SUM in Django ORM?

Question

Prologue:

This is a question arising often in SO:

• Django Models Group By
• Django equivalent for count and group by
• How to query as GROUP BY in django?
• How to use the ORM for the equivalent of a SQL count, group and join query?

I have composed an example on SO Documentation but since the Documentation will get shut down on August 8, 2017, I will follow the suggestion of this widely upvoted and discussed meta answer and transform my example to a self-answered post.

Of course, I would be more than happy to see any different approach as well!!

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Question:

Assume the model:

class Books(models.Model):     title  = models.CharField()     author = models.CharField()     price = models.FloatField() 

How can I perform the following queries on that model utilizing Django ORM:

• GROUP BY ... COUNT:

  SELECT author, COUNT(author) AS count FROM myapp_books GROUP BY author 

• GROUP BY ... SUM:

  SELECT author,  SUM (price) AS total_price FROM myapp_books GROUP BY author 

Answer 1

We can perform a GROUP BY ... COUNT or a GROUP BY ... SUM SQL equivalent queries on Django ORM, with the use of annotate(), values(), the django.db.models's Count and Sum methods respectfully and optionally the order_by() method:

• GROUP BY ... COUNT:

   from django.db.models import Count   result = Books.objects.values('author')                        .order_by('author')                        .annotate(count=Count('author')) 

  Now result contains a dictionary with two keys: author and count:

     author    | count  ------------|-------   OneAuthor  |   5  OtherAuthor |   2     ...      |  ... 

• GROUP BY ... SUM:

   from django.db.models import Sum    result = Books.objects.values('author')                         .order_by('author')                         .annotate(total_price=Sum('price')) 

  Now result contains a dictionary with two columns: author and total_price:

     author    | total_price  ------------|-------------   OneAuthor  |    100.35  OtherAuthor |     50.00      ...     |      ... 

UPDATE 13/04/2021

As @dgw points out in the comments, in the case that the model uses a meta option to order rows (ex. ordering), the order_by() clause is paramount for the success of the aggregation!