Django Frozen Data

Django model custom field for storing a frozen snapshot of an object.

Principles

• Behaves like a ForeignKey but the data is detached from the related object
• Transparent to the client - it looks and behaves like the original object
• The frozen object cannot be resaved
• Supports nesting of objects

Usage

A frozen field can be declared like a ForeignKey:

class Foo:
    frozen_bar = FrozenObjectField(Bar, help_text="This is a frozen snapshot of the object.")
    fresh_bar = ForeignKey(Bar, help_text="This is a live FK relationship.")

The field behaves exactly like a FK, with the exception that the object cannot be saved:

>>> bar = Bar()
>>> foo = Foo.objects.create(frozen_bar=bar, fresh_bar=bar)
>>> # the fresh field can be updated as you would expect
>>> foo.fresh_bar.save()
>>> # the frozen field cannot - to prevent overwriting new data.
>>> foo.frozen_bar.save()
>>> StaleObjectError: 'Object was frozen; defrosted objects cannot be saved.'

Issues - TODO

• Deserialization of DateField/DateTimeField values
• Deserialization of DecimalField values
• Deserialization of UUIDField values
• Deep object freezing

Running tests

The tests themselves use pytest as the test runner. If you have installed the poetry evironment, you can run them thus:

$ poetry run pytest