Get the value to filter on

        :return: the value to filter on

def value(self):
        """Get the value to filter on

        :return: the value to filter on
        """
        if self.filter_.get('field') is not None:
            try:
                result = getattr(self.model, self.filter_['field'])
            except AttributeError:
                raise InvalidFilters("{} has no attribute {}".format(self.model.__name__, self.filter_['field']))
            else:
                return result
        else:
            if 'val' not in self.filter_:
                raise InvalidFilters("Can't find value or field in a filter")

            return self.filter_['val']