Non-database field in Django model - django

Question

Is it possible to have a field in a Django model which does not get stored in the database.
For example:
class Book(models.Model):
title = models.CharField(max_length=75)
description models.CharField(max_length=255, blank=True)
pages = models.IntegerField()
none_db_field = ????
I could then do
book = Book.objects.get(pk=1)
book.none_db_field = 'some text...'
print book.none_db_field
Thanks

Answer 1

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

Answer 2

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'

Answer 3

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

Answer 4

If you want i18n support:
# Created by BaiJiFeiLong#gmail.com at 2022/5/2
from typing import Optional
from django.db import models
from django.utils.translation import gettext_lazy as _
class Blog(models.Model):
title = models.CharField(max_length=128, unique=True, verbose_name=_("Title"))
content = models.TextField(verbose_name=_("Content"))
_visitors: Optional[int] = None
#property
def visitors(self):
return self._visitors
#visitors.setter
def visitors(self, value):
self._visitors = value
visitors.fget.short_description = _("Visitors")