Using functions in models to dynamically fill fields on object creation in Django

Question

Within Django, can a function local to a model be called on object creation to dynamically fill a field?

class someModel(models.Model):
    id = models.FloatField(primary_key=True, default=generate_id())

    def generate_id(self):
        newId=time.time()
        return newId

So far I haven't had any luck with getting it to work or with locating documentation that covers it.

Answer 1

Yeah, this can be done using the signals framework.

from django.db import models
from django.core.signals import post_init

class SomeModel(models.Model):
    pass # *snip*

def generate_id(sender, **kwargs):
    kwargs.get('instance').newId = time.time()
        # ...

post_init.connect(SomeModel.generate_id)

Answer 2

from django.db import models
import time

class SomeModel(models.Model):
    id = models.FloatField(primary_key=True)

    def save(self, *args, **kwargs):
       if not self.id:
           self.id = self.generete_id()
       super(SomeModel, self).save(*args, **kwargs)

    def generate_id(self):
       return time.time()

Also be warned that there may be some code in Django internals that relies on the fact that unsaved model instance has no primary key (at least it was the case in v1.0).

Another pitfall is generating id by calling time.time(). Chanses are that you'll get duplicate ids under heavy load. I'd suggest using uuid module to generate unique identifiers.