Django Stale Fields

Tracking changed fields on a Django model instance.

Makes a Mixin available that will give you the properties:

• is_stale
• stale_fields

As well as the methods:

• save_stale()

Which will will selectively only update stale columns using the familiar Model.objects.filter(pk=pk).update(**stale_fields) pattern (but still resolves F() or auto_now constructs).

Installing

Install the package using pip. Then use the instructions in "Using the Mixin in the Model".

$ pip install django-stalefields

or if you're interested in developing it, use virtualenv and virtualenvwrapper. The default settings.py will look for the stalefields package in its current location.

$ mkvirtualenv django-stalefields
(django-stalefields)$ pip install -r example_app/requirements.pip
(django-stalefields)$ example_app/manage.py test testing_app

Auto-Mixin For All Models

You need to set make two settings.py tweaks:

# settings.py


INSTALLED_APPS = (
    'stalefields', # must be first!

    # the rest...
)

AUTO_STALE_FIELDS = True

This provides the methods and functionality automatically for all registered models.

Manual Mixin in the Model

from django.db import models
from stalefields import StaleFieldsMixin

class TestModel(StaleFieldsMixin, models.Model):
    """A simple test model to test stale fields mixin with"""
    boolean = models.BooleanField(default=True)
    characters = models.CharField(blank=True, max_length=80)

Using it in the shell

(django-stalefields)$ ./manage.py shell
>>> from testing_app.models import TestModel
>>> tm = TestModel(boolean=True, characters="testing")
>>> tm.save()
>>> tm.is_stale
False
>>> tm.stale_fields
()
>>> tm.boolean = False
>>> tm.is_stale
True
>>> tm.stale_fields
('boolean', )
>>> tm.characters = "have changed"
>>> tm.is_stale
True
>>> tm.stale_fields
('boolean', 'characters', )
>>> tm.save_dirty() # just saves the dirty fields via .update()
>>> tm.is_stale
False
>>> tm.get_stale_fields
()

Why would you want this?

Three reasons:

• Convenience
• Optimization
• Bug avoidance

When using signals, especially pressave, it is useful to be able to see what fields have changed or not. A signal could change its behaviour depending on whether a specific field has changed, whereas otherwise, you only could work on the event that the model's save() method had been called.

Any time you call boring old save() inside Django, all columns are inserted once again, which can be very heavy if, for example, you have lots of text in one column or many indexes that don't need to be needlessly checked for updating. Only updating changing columns via update() is much faster, but requires lots of state monitoring of your own accord. Put simply, this is nicer you your database!

Finally, if multiple threads call save() for different operations, only the more recent thread wins. If they both INSERTED only their column's changing values, that wouldn't be an issue! These kinds of bugs are a nightmare to chase down...

Credits

This code has largely be adapted from what was made available at Stack Overflow and adapted from Forked from https://github.com/smn/django-dirtyfields and https://github.com/callowayproject/django-dirtyfields..