python - 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!!

---

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!