def parse_storage_mappings(storage_mappings):
    """ Given the 'storage_mappings' API field, returns a tuple with the
    'default' option, the 'backend_mappings' and 'disk_mappings'.
    """
    # NOTE: the 'storage_mappings' property is Nullable:
    if storage_mappings is None:
        return None, {}, {}

    backend_mappings = {
        mapping['source']: mapping['destination']
        for mapping in storage_mappings.get("backend_mappings", [])}
    disk_mappings = {
        mapping['disk_id']: mapping['destination']
        for mapping in storage_mappings.get("disk_mappings", [])}

    return (
        storage_mappings.get("default"), backend_mappings, disk_mappings)