one_session_per_user.middleware.py

from one_session_per_user.models import User, Visitor
from django.contrib.sessions.models import Session


class OneSessionPerUserMiddleware(object):
    """http://stackoverflow.com/a/1814797"""

    def process_request(self, request):
        if isinstance(request.user, User):
            current_key = request.session.session_key
            if hasattr(request.user, 'visitor'):
                active_key = request.user.visitor.session_key
                print active_key, current_key
                if active_key != current_key:
                    Session.objects.filter(session_key=active_key).delete()
                    request.user.visitor.session_key = current_key
                    request.user.visitor.save()
            else:
                Visitor.objects.create(
                    pupil=request.user,
                    session_key=current_key,
                )

one_session_per_user.models.py

class Visitor(models.Model):
    pupil = models.OneToOneField(User, null=False)
    session_key = models.CharField(null=False, max_length=40)