How to auto generate a field value with the input field value in Django Model?

Question

I'm new to Django. I want to make an API that automatically generate a value to a field when POST to this api with Django.

def gen_code(k):
    return # a value
class generate_code(models.Model):
    code = models.CharField(max_length=100, default="", unique=True)
    generated_code = gen_code(code value) #generated with the value in code field

Something like the code above. How can I make that

Answer 1

You cannot do this in the declaration of the fields, as there is no "instance" at that point in time. You can override the default save() method to accomplish this:

class GeneratedCode(models.Model):
    code = models.CharField(max_length=100, default="", unique=True)
    generated_code = models.CharField(max_length=100, null=True)

    def save(self, *args, **kwargs):
        if not self.generated_code:
            self.generated_code = gen_code(self.code)
        
        super().save(*args, **kwargs)

This isn't perfect, but it should get you started. This code assumes that you always want to create a generated_code if it doesn't already exist, and you always want to generate it based on the current instance's "code" field.

It's also not a great idea to have a "default" value combined with unique=True. This will open you up to Unique constraint errors rather than having django complain that the field is mandatory - which it handles gracefully.