Django - Query with extra method that use python Function

Question

I would like to make a query in Django that use a function that take parameters from db e from python program. The function is shown below:

> def myFunc(a, b):
>     val = mathematical expression with a and b
>     return val

I would like that:

a comes from myTable (represent the field 'num')
b comes from python Code

class myTable(models.Model):
   num = models.IntegerField(default=0)

I would like to do a query similar to this in views.py:

b = 4     #it must be variable
c = 10    #it must be variable
myTable.objects.all().extra(select = {'new_param' : myFunc(num , b)}, where = ['new_param > %d'], params = [c])

How can I do that?

You can't run arbitrary python code as part of a query, if it's some mathematical expression it is probably possible using database functions. Can you share the actual contents of myFunc? — Iain Shelvington
– Iain Shelvington, Commented Aug 9, 2022 at 12:34
An example of myFunc() can be def myFunc(a,b): val = numpy.arcsin(math.sqrt( ((math.cos(a) * math.sin(abs(b - a))) ** 2) + 1) retun val — sablam
– sablam, Commented Aug 9, 2022 at 12:43
The function works correctly, and b is passed correctly as well. The problem is to pass the 'num' value of the database to the function. Do you know another way? — sablam
– sablam, Commented Aug 9, 2022 at 12:49

Iain Shelvington · Accepted Answer · 2022-08-09 12:56:07Z

1

You'll need to change your function to be an annotation using database functions because you can't pass arbitrary python code in a query to run on the database. Something like the following should be close to the expression inn your comment although it was a bit hard to read and is a bit complex

from django.db.models import F, Value
from django.db.models.functions import Cos, Sin, Abs, Power, Sqrt, ASin
b = 4
c = 10
myTable.objects.annotate(
    new_param=ASin(Sqrt(Power(Cos(F('num')) * Sin(Abs(Value(b) - F('num')))), Value(2)) + Value(1))
).filter(new_param__gt=c)

answered Aug 9, 2022 at 12:56

Iain Shelvington

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1 Comment

sablam Over a year ago

Using F(' ') the query works also using myFunc(), the function that I wrote. F(' ') was what was missing. Thank you very much.

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