Immutable Django model fields

22 June 2008 — django

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I wanted to make a field on a Django model read-only after it was assigned an initial value. Maybe I was having a slow day, or my Google-fu was especially lacking, but I couldn't find a documented way to do this. There's the editable Field option, but I wanted to ensure I couldn't mistakenly overwrite the initial value anywhere — not just in the admin interface or form processing. It turned out to be pretty easy.

The solution was to override the Model's __setattr__ method and intercept writes for the Field in question. Here's an example Model:

def random_hash(): 
  digest = hashlib.sha512()
  digest.update("%s%s%s" % (settings.SECRET_KEY, time.time(), random.random()))
  l = long(digest.hexdigest(), 16)
  return struct.pack('d', l).encode('base64').replace('\n', '').strip('=')

class Order(models.Model):
  confirmation_id = models.CharField(max_length=128, default=random_hash)

  def __setattr__(self, name, value):
    if name == 'confirmation_id':
        if getattr(self, 'confirmation_id', None):
            return
    super(Order, self).__setattr__(name, value)

  def __unicode__(self):
    return u"Order %s, confirmation_id: %s" % (self.id, self.confirmation_id)

Not elegant, but it works:

>>> o = Order()
>>> o.save()
>>> o
<Order: Order 4, confirmation_id: vzb0jXVz6l8>
>>> o.confirmation_id = 'something else'
>>> o
<Order: Order 4, confirmation_id: vzb0jXVz6l8>
>>>

A more general solution would be a Model subclass with a list of the immutable fields. You can't put that in the model's Meta class (see this ticket), so here's what I use:

from django.db import models

class ImmutableModel(models.Model):
    immutable_fields = []
    class Meta:
        abstract = True

    def __setattr__(self, name, value):
        if name == 'immutable_fields' or name in self.immutable_fields:
            if getattr(self, name, None):
                return
        super(ImmutableModel, self).__setattr__(name, value)

Make your model a subclass of that and set the immutable_fields attribute to a list of fields that should be immutable once set. Good enough to protect you from yourself unless you're really trying.

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