settings_singleton_model_example

settings_singleton_model_example.py

# models.py
# the dev adds new fields to this as the client asks for more features
class ClientSettings(models.Model):
  base_labour_rate = models.FloatField(...)


# middleware.py
class ClientSettingsMiddleware(object):
    _settings = None

    def __init__(self, get_response):
        self.get_response = get_response

    def __call__(self, request):
        if not self._settings:
            # there should only ever be a single ClientSettings instance/record 
            # in the DB; cache it in this middleware to prevent unnecessary DB 
            # lookups
            self._settings = ClientSettings.objects.order_by('pk').first()        
            
        # set the settings on `request` so it's accessible from everywhere;
        # in views, etc where ever `request` is passed
        setattr(request, 'client_settings', self._settings)

        response = self.get_response(request)

        # set it up so that if the user changes the settings via 
        # admin, it sets a response attribute to cause this middleware 
        # to reload it's cached settings attribute
        if getattr(response, 'reload_client_settings', False):
            self._settings = None

        return response