django-immutablefield

Inspired by a Google search that didn't turn up reusable solution for making fields immutable inside of a Django model.

Installing

One of the following:

Via the ole' standby:

easy_install django-immutablefield

Pip:

pip install django-immutablefield

To install directly from Bitbucket:

pip install -e hg+http://bitbucket.org/robmadole/django-immutablefield#egg=django-immutablefield

Hint

You do not need to add anything into Django's INSTALLED_APPS

What does it do

Allows you to define certain fields as immutable inside of Django models.

It works as a drop-in replacement for Django's own Model. This means you can ImmutableModel even if you don't specify ImmutableMeta.

from django.db import models

from immutablefield.models import ImmutableModel

CruiseShip(ImmutableModel):
    name = models.CharField(max_length=50)

    class ImmutableMeta:
        # After ya name a ship, you can't change it matey
        immutable = ['name']

    def __unicode__(self):
        return u'%s' % self.name

Now you can try with all your might, but the field won't change (within reason, sure this is Python we can do almost anything if we try hard enough)

>>> queen_anne = CruiseShip.objects.create(name='Queen Anne')
<CruiseShip 'Queen Anne'>
>>> queen_anne.name = 'King George'
>>> queen_anne.name
'Queen Anne'

You can make it complain

Change the meta section to include quiet = False and it will raise a ValueError if an attempt is made to change this value

class ImmutableMeta:
    # After ya name a ship, you can't change it matey
    immutable = ['name']
    quiet = False

The error is raised as soon as you try and set the field, not when save() is called.

>>> queen_anne = CruiseShip.objects.create(name='Queen Anne')
<CruiseShip 'Queen Anne'>
>>> queen_anne.name = 'King George'
ValueError: name is immutable and cannot be changed

Reference

ImmutableMeta

Specify options that control how immutable fields are handled when subclassing the ImmutableModel class

immutable
Tell ImmutableModel which fields should not be allowed to change. This value must be a tuple or a list and contain the names of the fields as strings.:
class ImmutableMeta:
    immutable = ['my_special_id']
Specify multiple fields:
class ImmutableMeta:
    immutable = ['my_special_id', 'name', 'foreign_key']
quiet
If an attempt is made to change an immutable field, should we quietly prevent it.

Set this value to False to raise a ValueError when an immutable field is changed.:
class ImmutableMeta:
    immutable = ['my_special_id']
    quiet = False