Django Tip: Non-editable Fields

Yesterday I explained how to subclass a DateTimeField, adding a pre_save() method to make it suitable for use as a row creation or modification timestamp. If you do this, however, you will observe that unless you add an editable=False argument to the field instantiation call they will appear as editable fields in any ModelForms containing them. It would clearly be more desirable to have these fields non-editable by default, which can easily be arranged by extending their __init__() method and changing the default value of the editable argument.

The code below also renames the fields more suitably. You can use a similar trick to create other non-editable field types, and also to change other instantiation defaults.

class ModificationDateTimeField(models.DateTimeField):
  def __init__(self, editable=False, *args, **kw):
      models.DateTimeField.__init__(self, editable=editable, *args, **kw)
  def pre_save(self, instance, add):
      val = datetime.datetime.now()
      setattr(instance, self.attname, val)
      return val

class CreationDateTimeField(ModificationDateTimeField):
  def __init__(self, editable=False, *args, **kw):
      models.DateTimeField.__init__(self, editable=editable, *args, **kw)
  def pre_save(self, instance, add):
      if not add:
          return getattr(instance, self.attname)
      return ModificationDateTimeField.pre_save(self, instance, add)