Non-database field in Django model

To make it an instance variable (so each instance gets its own copy), you'll want to do this

class Book(models.Model):
    title = models.CharField(max_length=75)
    #etc

    def __init__(self, *args, **kwargs):
        super(Foo, self).__init__(*args, **kwargs)
        self.editable = False

Each Book will now have an editable that wont be persisted to the database

As long as you do not want the property to persist, I don't see why you can't create a property like you described. I actually do the same thing on certain models to determine which are editable.

class Email(EntryObj):
    ts = models.DateTimeField(auto_now_add=True)
    body = models.TextField(blank=True)
    user = models.ForeignKey(User, blank=True, null=True)
    editable = False
    ...


class Note(EntryObj):
    ts = models.DateTimeField(auto_now_add=True)
    note = models.TextField(blank=True)
    user = models.ForeignKey(User, blank=True, null=True)
    editable = True

Creating a property on the model will do this, but you won't be able to query on it.

Example:

from django.db import models

class Person(models.Model):
    first_name = models.CharField(max_length=30)
    last_name = models.CharField(max_length=30)

    def _get_full_name(self):
        return "%s %s" % (self.first_name, self.last_name)

    def _set_full_name(self, combined_name):
        self.first_name, self.last_name = combined_name.split(' ', 1)

    full_name = property(_get_full_name)

    full_name_2 = property(_get_full_name, _set_full_name)

Usage:

from mysite.models import Person

a = Person(first_name='John', last_name='Lennon')
a.save()
a.full_name
'John Lennon'

# The "full_name" property hasn't provided a "set" method.
a.full_name = 'Paul McCartney'
Traceback (most recent call last):
    ...
AttributeError: can't set attribute

# But "full_name_2" has, and it can be used to initialise the class.
a2 = Person(full_name_2 = 'Paul McCartney')
a2.save()
a2.first_name
'Paul'