Question

I want to log actions made by users. In most OO languages, I would implement this via a LoggedAction class, having several child classes like LoginActionand LogoutAction. I could then iterate over a list of LoggedActions and get the specific child behaviour through virtual inheritance. This does not work using Django models however.

Example models.py:

class LoggedAction(models.Model):
    user = models.ForeignKey(User)
    timestamp = models.DateTimeField(auto_now_add=True)

    def __unicode__(self):
        return "%s: %s %s" % (unicode(self.timestamp), unicode(self.user), unicode(self.action()))

    def action(self):
        return ""

class LoginAction(LoggedAction):
    def action(self):
        return "logged in"

class LogoutAction(LoggedAction):
    def action(self):
        return "logged out"

Then I’d like to do [unicode(l) for l in LoggedAction.objects.all()] and get a list of messages like u'2012-02-18 18:47:09.105840: knatten logged in'.

As expected, this does not work, since what I get from all() is a list of LoggedAction objects having either a loginaction member or a logoutaction member. (The output is a list of messages like u'2012-02-18 18:47:09.105840: knatten, with no mention of the action.)

Is there a sane way to get the behaviour I’m after, or am I trying to apply the wrong paradigm here? (I guess I am, and that I should just have the specific action as a member in LoggedAction)

Answer 1

Yes, this is probably the wrong paradigm. It’s easy to be misled by the object-relational mapper (ORM) – database tables don’t really map all that well to objects, and this difference is known as the object-relational impedance mismatch.

What you actually need is to make action a field. This field can take a choices parameter which represents the possible values of that field – ie logged in or logged out:

class LoggedAction(models.Model):
    ACTIONS = (
       ('I', 'logged in'),
       ('O', 'logged out')
    )
    user = models.ForeignKey(User)
    timestamp = models.DateTimeField(auto_now_add=True)
    action = models.CharField(max_length=1, choices=ACTIONS)

    def __unicode__(self):
        return u"%s: %s %s" % (self.timestamp, self.user, self.get_action_display())

Note that I’ve used arbitrary single-character strings to represent the actions, and the get_action_display() magic method to get the full description.