Last Updated: February 25, 2016
·
6.252K
· __imom0

Dark cornors of Django's auto_now_add

There is a model declaration in one of our existing projects,

class Event(models.Model):
    name = models.CharField(max_length=20)
    created_at = models.DateTimeField(auto_now_add=True)

We use auto_now_add for specifying the created time automatically.When the Event model is used more and more, sometimes we have scenarios that created_at is not equal to the created time of model instance.

event = Event(name='new event')
# today is 2013-08-24
event.created_at = datetime.datetime(2013, 8, 10)
event.save()
>>> event.created_at
datetime.datetime(2013, 8, 24, 14, 10, 11, 181193)

It's not expected, but the django docs explain the rule:

Note that the current date is always used; it’s not just a default value that you can override.

When you want set the datetime to what you want, just save again,

event.created_at = datetime.datetime(2013, 8, 10)
event.save()

Then I add a field default value to replace auto_now_add:

import datetime

class Event(models.Model):
    name = models.CharField(max_length=20)
    created_at = models.DateTimeField(default=datetime.datetime.now)

But this is not completely same effect as auto_now_add, a class based CreateView of Event gets involved into a form validation error created_at is needed.To solve this problem, add blank=True to created_at field is working but not good enough.The best way is to create a ModelForm with only fields related.

from django import forms

class EventForm(forms.ModelForm):
    class Meta:
        model = Event
        fields = ('name',)

class EventCreateView(CreateView):
    model_form = EventForm

The conclusion I made from this is that using Form to handle user's posted form data is always a good practice even the Form looks like having done nothing.