Generate Random String for Model Field in Django

Question

I am using DRF and have a couple models where I want to generate unique numbers separate from ID's. For example, one model would have a number like PRJ023400 and another would be TSK2949392.

I created a utility function to generate random numbers and as the field and attempted to use this function as the default.

number = models.CharField(max_length = 10,
                          blank=True,
                          editable=False,
                          default='PRJ'+create_new_ref_number())

The problem I am facing is that this only generates a new string when running a migration. What would be the best approach to make this work?

Cheers!

Abdul Aziz Barkat · Accepted Answer · 2021-02-09 16:34:00Z

3

default can accept a function. As per the documentation:

The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.

Considering all this you can set it to a function like this:

def number_default_function():
    return 'PRJ' + create_new_ref_number()

class YourModel(models.Model):
    number = models.CharField(
        max_length = 10,
        blank=True,
        editable=False,
        default=number_default_function
    )

answered Feb 9, 2021 at 16:34

Abdul Aziz Barkat

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2 Comments

JayG.Dev Over a year ago

I tried this out but unfortunately it returned the same results.

Abdul Aziz Barkat Over a year ago

@PixelsOfMind your previous approach failed because default='PRJ'+create_new_ref_number() was evaluated when the line was evaluated, i.e. default was set to 'PRJ<SOMETHING>', If the approach in my answer fails that means that your function create_new_ref_number returns the same thing for some reason.

JayG.Dev · Accepted Answer · 2021-02-10 16:38:37Z

1

I was actually able to figure this out.

For the model field, I changed it to:

number = models.CharField(max_length = 10,
                              blank=True,
                              null=True,
                              editable=False)

When saving, I added this function to generate a new string each time:

def save(self, *args, **kwargs):
        self.number = 'PRJ' + create_new_ref_number()
        super(Project, self).save(*args, **kwargs)

Hopefully this can help someone else out.

Cheers.

answered Feb 10, 2021 at 16:38

JayG.Dev

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1 Comment

jma Over a year ago

In passing, it's worth being careful if number is user-visible, since it leaks information about how many objects you have and the rate of object creation. That may not be desirable.

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