Returns a cleaned and validated value. Raises a ValidationError
        if there's a problem

def validate(self, value):
        """
        Returns a cleaned and validated value. Raises a ValidationError
        if there's a problem
        """
        if value is None:
            if self.required:
                raise ValidationError('{} - None values are not allowed'.format(self.column_name or self.db_field))
        return value