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In progress: Making a version of django-immutablefield that is more about assuming immutability, pulling in Helder Silva's changes too.
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README.rst

django-immutablemodel

Easily make all of a django model's fields immutable after saving. You can customize it to make some fields mutable. You can change when it can be mutable (e.g. whenever the field's value is nil, or only when a particular 'lock field' is false)

History and credits

Very heavily based on Rob Madole's original code at https://bitbucket.org/robmadole/django-immutablefield and Helder Silva's fork at https://bitbucket.org/skandal/django-immutablefield. Rob's was 'inspired by a Google search that didn't turn up reusable solution for making fields immutable inside of a Django model'.

Pulled across from hg/bitbucket to git/github because less headache as they are more familar to me (Tim)

Installing

One of the following:

Via the ole' standby:

easy_install django-immutablemodel

Pip:

pip install django-immutablemodel

To install directly from Github:

pip install git+https://github.com/red56/django-immutablemodel

Hint

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

What does it do

Allows you to declare a Django model as immutable.

It works as a drop-in replacement for Django's own Model. This means you can ImmutableModel.

from django.db import models

from immutablemodel.models import ImmutableModel

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

    class Meta:
        mutable_fields = [] # you can actually leave this out...

Now you can try with all your might, but once you've saved it 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 immutable_quiet = False and it will raise a ValueError if an attempt is made to change this value

class Meta:
    mutable_fields = [] # you can actually leave this out...
    immutable_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

If you want you can make ALL immutable fields complain by adding IMMUTABLE_QUIET=False to your settings.py

You can make some fields mutable

List the fields you actually want mutable in "mutable_fields"

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

    class Meta:
         mutable_fields = ['passengers']

Please note that fields beginning with an underscore are ignored by ImmutableModel - this allows immutable_lock_field to be a @property (ie. they are automatically mutable - thanks to https://github.com/Bouke for contributing a patch for this -- see https://github.com/red56/django-immutablemodel/pull/1)

Reference

Meta

Specify options (in addition to the normal django model's Meta options) that control how immutable fields are handled when subclassing the ImmutableModel class

mutable_fields

Tell ImmutableModel which fields should be allowed to change. This value must be a tuple or a list and contain the names of the fields as strings.:

class Meta:
    mutable_fields = ['some_transient_data']

Specify multiple fields:

class ImmutableMeta:
    mutable_fields = ['some_transient_data', 'name', 'foreign_key']

immutable_fields

Tell ImmutableModel which fields should not be allowed to change. NB: you can't specify mutable_fields AND immutable_fields. This value must be a tuple or a list and contain the names of the fields as strings.:

class Meta:
    immutable_fields = ['my_special_id']

Specify multiple fields:

class ImmutableMeta:
    immutable_fields = ['my_special_id', 'name', 'foreign_key']

immutable_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_quiet = False

immutable_lock_field

This determines when to enforce immutability. By default it is equal to immutable_model.models.PK_FIELD. This means that when the PK_FIELD is full (typically when saved) the model is immutable, but before it is saved it is mutable. Alternatively you can specify a field by name, or you can set it to None, which means that you can't change immutable fields once they are set (even before saving).

class ImmutableMeta:
immutable_lock_field = ['is_locked']

settings.py

IMMUTABLE_QUIET

Set this to False to make all immutable_fields raise an Exception when attempting to be changed.
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