django-fsm adds declarative states managment for django models. Instead of adding some state field to a django model, and manage it values by hand, you could use FSMState field and mark model methods with the transition decorator. Your method will contain the side-effects of the state change.

The decorator also takes a list of conditions, all of which must be met before a transition is allowed.

$ python setup.py install

Or, for the latest git version

$ pip install -e git://github.com/kmmbvnr/django-fsm.git#egg=django-fsm

Add FSMState field to you model

from fsm.fields import FSMField

class BlogPost(models.Model):
    state = FSMField(default='new')

Use the transition decorator to annotate model methods

from fsm import transition

@transition(source='new', target='published')
def publish(self):
    """
    This function may contain side-effects, 
    like updating caches, notifying users, etc.
    The return value will be discarded.
    """

source parameter accepts a list of states, or an individual state. You can use * for source, to allow switching to target from any state.

If calling publish() succeeds without raising an exception, the state field will be changed, but not written to the database.

from fsm import can_proceed

def publish_view(request, post_id):
    post = get_object__or_404(BlogPost, pk=post_id)
    if not can_proceed(post.publish):
         raise Http404;

    post.publish()
    post.save()
    return redirect('/')

If you use the transition decorator with the save argument set to True, the new state will be written to the database

@transition(source='new', target='published', save=True)
def publish(self):
    """
    Side effects other than changing state goes here
    """

If you require some conditions to be met before changing state, use the conditions argument to transition. conditions must be a list of functions that take one argument, the model instance. The function must return either True or False or a value that evaluates to True or False. If all functions return True, all conditions are considered to be met and transition is allowed to happen. If one of the functions return False, the transition will not happen. These functions should not have any side effects.

You can use ordinary functions

def can_publish(instance):
    # No publishing after 17 hours
    if datetime.datetime.now().hour > 17:
       return False
    return True

Or model methods

def can_destroy(self):
    return self.is_under_investigation()

Use the conditions like this:

@transition(source='new', target='published', conditions=[can_publish])
def publish(self):
    """
    Side effects galore
    """

@transition(source='*', target='destroyed', conditions=[can_destroy])
def destroy(self):
    """
    Side effects galore
    """

If you store the states in the db table you could use FSMKeyField to ensure Foreign Key database integrity.