Django Casting Character as Integer Query

Question

I have a simple model similar to the model below:

class ServiceIncoice(MyCustomRegistrationModel):
    (...)
    number = models.CharField(max_length=60)
    (...)

The field number contains integer values generally. The user might want to use only numbers or alphanumerical codes.

The problem is I need to get the last value of the field number when the user wants to create a new ServiceInvoice instance and then add 1 to the last value. For example:

Id = 1, Number = 13
Id = 2, Number = 2
Id = 3, Number = 16

The next item should have Id = 4 and Number = 17. However when I execute

ServiceInvoice.objects.all().latest('number')

It returns 2 which isn't what I want. I have tried this solution, this one and this one. Anyone of them worked for me because I need the highest number of any given ServiceInvoice.

Thanks in advance.

Thanks for pointing me in the right direction. The first solution is the one I needed. — meshantz, Apr 05 '16 at 17:04
Hi @meshantz I'm happy for helping you out. Could you consider marking it as useful answer? Cheers. — tiagomenegaz, Apr 06 '16 at 21:04
I voted up the question. You haven't selected a preferred answer, and you can't vote for your own answer :) — meshantz, Apr 08 '16 at 14:26

score 0 · Answer 1 · answered Oct 15 '15 at 18:49

0

According to django docs.

Latest Returns the latest object in the table, by date, using the field_name provided as the date field.

You can do

qs = ServiceInvoice.objects.all().order_by('-number')[:1]

answered Oct 15 '15 at 18:49

Rizwan Mumtaz

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I tried your solution @Rizwan but it comes up with the same result. I think I need to cast number as integer because it's stored as char field (number = models.CharField). In char fields the order is like 1, 11, 111, ..., 2, 21, 211 and so on. Thank you. – tiagomenegaz Oct 15 '15 at 19:07

score 0 · Answer 2 · edited May 23 '17 at 12:15

0

The first solution you referenced should work as long as you update the cast in your extra call to work for your database implementation. For postgresql, this should look like:

ServiceInvoice.objects.extra(
    {'number_as_int': "number::INTEGER"}
).latest('number_as_int')

edited May 23 '17 at 12:15

Community

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answered Apr 05 '16 at 17:04

meshantz

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Django Casting Character as Integer Query

2 Answers2