Value variables not supported for predicate with list type.

Error message

Value variables not supported for predicate with list type.

What it means

Value variables (val(x)) capture a single value per source UID. When the predicate has a list type, a node can hold multiple values, so Dgraph refuses to materialize a value variable over it and throws this error when it detects more than one value in a valueMatrix cell.

Source

Thrown at query/query.go:1640

		}

		// For a recurse query this can happen. We don't allow using the same variable more than
		// once otherwise.
		lists := append([]*pb.List(nil), v.Uids, uids)
		v.Uids = algo.MergeSorted(lists)
		doneVars[sg.Params.Var] = v
	case len(sg.valueMatrix) != 0 && sg.SrcUIDs != nil && len(sgPath) != 0:
		// 4. A value variable. We get the first value from every list thats part of ValueMatrix
		// and store it corresponding to a uid in SrcUIDs.
		if v, ok = doneVars[sg.Params.Var]; !ok {
			v.Vals = types.NewShardedMap()
			v.path = sgPath
			v.strList = sg.valueMatrix
		}

		for idx, uid := range sg.SrcUIDs.Uids {
			if len(sg.valueMatrix[idx].Values) > 1 {
				return errors.Errorf("Value variables not supported for predicate with list type.")
			}

			if len(sg.valueMatrix[idx].Values) == 0 {
				continue
			}
			val, err := convertWithBestEffort(sg.valueMatrix[idx].Values[0], sg.Attr)
			if err != nil {
				continue
			}
			v.Vals.Set(uid, val)
		}
		doneVars[sg.Params.Var] = v
	default:
		// If the variable already existed and now we see it again without any DestUIDs or
		// ValueMatrix then lets just return.
		if _, ok := doneVars[sg.Params.Var]; ok {
			return nil
		}

View on GitHub (pinned to 759e242be6)

Solutions

Check the schema (/state) — if the predicate is a list type, do not capture it into a value variable
Change the predicate schema to scalar if single-valued semantics are intended (requires data migration)
Use math/aggregation over the list via supported functions (e.g. len()) instead of value vars
Use @groupby or an aggregation without value-variable capture

Example fix

// before: tags is [string]
{
  me(func: uid(1)) { t as tags }
  other(func: uid(2)) { uses(val(t)) }
}
// after: use a scalar predicate
{
  me(func: uid(1)) { t as primary_tag }
  other(func: uid(2)) { uses(val(t)) }
}

Defensive patterns

Strategy: validation

Validate before calling

// Before capturing a predicate into a value variable, confirm it is not a list type
if schemaType(pred) == "list" {
    return fmt.Errorf("cannot use value variable on list-type predicate %s", pred)
}

Type guard

func isScalarPredicate(t string) bool { return t == "string" || t == "int" || t == "float" || t == "bool" || t == "datetime" || t == "uid" }

Try / catch

if err != nil && strings.Contains(err.Error(), "Value variables not supported for predicate with list type") {
    return fmt.Errorf("predicate is a list type; use len() or aggregation instead: %w", err)
}

Prevention

Check /state or schema JSON before writing value-variable queries
Keep list and scalar predicates under distinct, clearly named predicates
Document which predicates are lists and forbid val() capture in query templates

When it happens

Trigger: Using 'as var' / val() on a predicate whose schema type is [type] (a list), and a node has 2+ values for that predicate; e.g. 'friend { tags as tags }' where tags is a list.

Common situations: Schema defines the predicate as a list but the query treats it as scalar for variable capture; data written to a predicate later migrated to list type; queries copied from scalar predicates to list predicates.

Related errors

AI-assisted analysis of dgraph-io/dgraph@759e242be6 (2026-09-01). Data as JSON: /api/errors/01e05c6c8caa090f. Report an issue: GitHub.