Function sortByValue

worker/sort.go:775–821

sortByValue fetches values and sort UIDList.

(ctx context.Context, ts *pb.SortMessage, ul *pb.List,
	typ types.TypeID)

// sortByValue fetches values and sort UIDList.
func sortByValue(ctx context.Context, ts *pb.SortMessage, ul *pb.List,
	typ types.TypeID) ([]types.Val, error) {
	lenList := len(ul.Uids)
	uids := make([]uint64, 0, lenList)
	values := make([][]types.Val, 0, lenList)
	multiSortVals := make([]types.Val, 0, lenList)
	order := ts.Order[0]

	var lang string
	if langCount := len(order.Langs); langCount == 1 {
		lang = order.Langs[0]
	} else if langCount > 1 {
		return nil, errors.Errorf("Sorting on multiple language is not supported.")
	}

	// nullsList is the list of UIDs for which value doesn't exist.
	var nullsList []uint64
	var nullVals [][]types.Val
	for i := range lenList {
		select {
		case <-ctx.Done():
			return multiSortVals, ctx.Err()
		default:
			uid := ul.Uids[i]
			val, err := fetchValue(uid, order.Attr, order.Langs, typ, ts.ReadTs)
			if err != nil {
				// Value couldn't be found or couldn't be converted to the sort type.
				// It will be appended to the end of the result based on the pagination.
				val.Value = nil
				nullsList = append(nullsList, uid)
				nullVals = append(nullVals, []types.Val{val})
				continue
			}
			uids = append(uids, uid)
			values = append(values, []types.Val{val})
		}
	}
	err := types.Sort(values, &uids, []bool{order.Desc}, lang)
	ul.Uids = append(uids, nullsList...)
	values = append(values, nullVals...)
	if len(ts.Order) > 1 {
		for _, v := range values {
			multiSortVals = append(multiSortVals, v[0])
		}
	}
	return multiSortVals, err
}

// fetchValue gets the value for a given UID.
func fetchValue(uid uint64, attr string, langs []string, scalar types.TypeID,