CODEWITHSUNDEEP
pythondjango
JD Gamboa
JD Gamboa

Reputation: 382

Django - aggregate and annotate for simple operations

I see in the docs the aggregate and annotate functions can be used to create a new column in the query, so if I write something like:

my_object = ...objects.filter(something).annotate(extra_column=Avg(value))

the inner query will give an extra column

, AVG(value) AS "extra_column" ...

Now, it seems to me that it can be used only will functions like count, avg, max, and min... can I do something as a simple +/- certain number?

I'm trying .annotate(extra_column=another_column+1) or .annotate(extra_column='another_column'+1) but it doesn't work.

What am I doing wrong? Sorry for the silly question.

Upvotes: 0

Views: 678

Answers (2)

Borut
Borut

Reputation: 3364

You'd use F() expressions for arithmetic. F() is part of query expressions. From the documentation:

Django supports addition, subtraction, multiplication, division, modulo arithmetic, and the power operator on query expressions, using Python constants, variables, and even other expressions.

A code example is:

from django.db.models import F

my_object = Model.objects.annotate(extra_column=F('another_column') + 1)
my_object = Model.objects.annotate(extra_column=F('another_column') * F('yet_another_column'))

Upvotes: 2

Laurynas Tamulevičius
Laurynas Tamulevičius

Reputation: 1579

The aggregate functions do return the integer which could be manipulated by adding, subtracting etc.

The following example works on Django 1.11.6:

from django.db.models import Count
employees = Employee.objects.annotate(managers=Count('manager') + 10) # every employee has only a single manager.
print employees[0].managers # prints out 11

Upvotes: 0

Related Questions