django-model-changes allows you to track the state and changes of a model instance:

1. Install django-model-changes:

   pip install django-model-changes
   

2. Add "django_model_changes" to your INSTALLED_APPS setting like this:

   INSTALLED_APPS = (
       ...
       'django_model_changes',
   )
   

3. Add the ChangesMixin to your model:

   >>> from django.db import models
   >>> from django_model_changes import ChangesMixin
   
   >>> class User(ChangesMixin, models.Model):
   >>>     name = models.CharField(max_length=100)
   

4. Get instance changes:

   >>> user = User()
   >>> user.name = 'Foo Bar'
   >>> user.save()
   
   >>> user.name 'I got a new name'
   
   >>> # Get current state
   >>> user.current_state()
   {'id': 1, 'name': 'I got a new name'}
   
   >>> # Get previous state (state after previous save/create/delete)
   >>> user.previous_state()
   {'id': 1, 'name': 'Foo Bar'}
   
   >>> # Get old state (state before previous save/create/delete)
   >>> user.old_state()
   {'id': None, 'name': ''}
   
   >>> # Get changes from the previous state to the current state
   >>> user.changes()
   {'name': ('Foo Bar', 'I got a new name')}
   
   >>> # Get changes from the old state to the current state
   >>> user.old_changes()
   {'id': (None, 1), 'name': ('', 'Foo Bar')}
   
   >>> # Check if the instance was persisted (saved)
   >>> user.was_persisted()
   False
   
   >>> # Check if the instance is persisted
   >>> user.is_persisted()
   True
   

5. Listen for changes:

   >>> from django_model_changes import post_change
   

   >>> def my_callback(sender, instance, **kwargs):
   >>>     # Do something with previous and current state
   >>>     instance.old_state()
   >>>     instance.current_state()

   >>>     # There is also a convenience method to get
   >>>     # an instance from the previous state
   >>>     instance.old_instance()

   >>> post_change.connect(my_callback, User)

django-model-changes allows you to retrieve the following states from an instance:

1. current_state()

   The current state of the instance.

2. previous_state()

   The state of the instance after it was created, saved or deleted the last time.

3. old_state()

   The previous previous_state(), i.e. the state of the instance before it was created, saved or deleted the last time.

It also provides convenience methods to get changes between states:

1. changes()

   Changes from previous_state to current_state.

2. previous_changes()

   Changes from old_state to previous_state.

3. old_changes()

   Changes from old_state to current_state.

And the following methods to determine if an instance was/is persisted in the database:

1. was_persisted()

   Was the instance persisted in its old state.

2. is_persisted()

   Is the instance is_persisted in its current state.

This schematic tries to illustrate how these methods relate to each other:

after create/save/delete            after save/delete                  now
|                                   |                                  |
.-----------------------------------.----------------------------------.
|\                                  |\                                 |\
| \                                 | \                                | \
|  old_state()                      |  previous_state()                |  current_state()
|                                   |                                  |
|-----------------------------------|----------------------------------|
|  previous_changes() (prev - old)  |  changes() (cur - prev)          |
|-----------------------------------|----------------------------------|
|                      old_changes()  (cur - old)                      |
.----------------------------------------------------------------------.
 \                                                                      \
  \                                                                      \
   was_persisted()                                                        is_persisted()

Refer to the doc strings in changes.py, or build the documentation:

>>> pip install Sphinx
>>> cd docs
>>> make html
Open build/html/index.html