How to execute arithmetic operations between Model fields in django

Question

Prologue:

This is a question arising often in SO:

• Subtracting two annotated columns
• Django query with simple arithmetic among

Answer 1

F() expressions can be used to execute arithmetic operations (+, -, * etc.) among model fields, in order to define an algebraic lookup/connection between them.

An F() object represents the value of a model field or annotated column. It makes it possible to refer to model field values and perform database operations using them without actually having to pull them out of the database into Python memory.

let's tackle the issues then:

• The product of two fields:

  result = MyModel.objects.all().annotate(prod=F('number_1') * F('number_2'))
  

  Now every item in result has an extra column named 'prod' which contains the product of number_1 and number_2 of each item respectively.

• Filter by day difference:

  from datetime import timedelta
  
  result = MyModel.objects.all().annotate(
               delta=F('date_2') - F('date_1')
           ).filter(delta__gte=timedelta(days=10))
  

  Now the items in result are those from MyModel whose date_2 is 10 or more days older than date_1. These items have a new column named delta with that difference.

• A different case:

  We can even use F() expressions to make arithmetic operations on annotated columns as follows:

  result = MyModel.objects.all()
                          .annotate(sum_1=Sum('number_1'))
                          .annotate(sum_2=Sum('number_2'))
                          .annotate(sum_diff=F('sum_2') - F('sum_1'))