Prepare model fields attribute.

def prepare_fields_attribute(attribute_name, attributes, class_name):
        """Prepare model fields attribute."""
        attribute = attributes.get(attribute_name)
        if not attribute:
            attribute = tuple()
        elif isinstance(attribute, std_collections.Iterable):
            attribute = tuple(attribute)
        else:
            raise errors.Error('{0}.{1} is supposed to be a list of {2}, '
                               'instead {3} given', class_name, attribute_name,
                               fields.Field, attribute)
        return attribute