What causes "Expected 1 child for not but got %d." in dgraph-io/dgraph?

Give `not` exactly one operand: @facets(not(eq(name, "x"))).

Expected 1 child for not but got %d.

Error message

Expected 1 child for not but got %d.

What it means

The facet filter tree builder validates boolean operator arity: a `not` node must have exactly one child. Receiving 0 or 2+ children under a not means the parsed query's boolean structure is malformed for facets.

Source

Thrown at worker/task.go:2483

		default:
			return nil, errors.Errorf("Fn %s not supported in preprocessFilter.", fname)
		}
		return ftree, nil
	}

	for _, c := range tree.Children {
		ftreec, err := preprocessFilter(c)
		if err != nil {
			return nil, err
		}
		ftree.children = append(ftree.children, ftreec)
	}

	numChild := len(tree.Children)
	switch ftree.op {
	case "not":
		if numChild != 1 {
			return nil, errors.Errorf("Expected 1 child for not but got %d.", numChild)
		}
	case "and":
		if numChild != 2 {
			return nil, errors.Errorf("Expected 2 child for not but got %d.", numChild)
		}
	case "or":
		if numChild != 2 {
			return nil, errors.Errorf("Expected 2 child for not but got %d.", numChild)
		}
	default:
		return nil, errors.Errorf("Unsupported operation in facet filtering: %s.", tree.Op)
	}
	return ftree, nil
}

type countParams struct {
	readTs  uint64
	counts  []int64

View on GitHub (pinned to 759e242be6)

Solutions

Give `not` exactly one operand: @facets(not(eq(name, "x"))).
Wrap a multi-condition negation in parentheses: @facets(not((eq(a,"1") OR eq(b,"2")))).
Check the query generator's parenthesis emission for NOT.

Example fix

// before
@facets(not(eq(a, "1") AND eq(b, "2")))
// after
@facets(not((eq(a, "1") AND eq(b, "2"))))

Defensive patterns

Strategy: validation

Validate before calling

function validateNotArity(tree) {
  if (tree.op === "not" && tree.children.length !== 1)
    throw new Error("not must have exactly 1 child in facet filter");
  (tree.children || []).forEach(validateNotArity);
}

Try / catch

try {
  return await txn.query(query);
} catch (e) {
  if (/Expected 1 child for not/.test(String(e))) {
    // fix boolean nesting in the query generator
  }
  throw e;
}

Prevention

Always wrap multi-operand negations in parentheses.
Ensure NOT nodes in query builders emit exactly one child.
Add AST-arity unit tests for NOT in generated queries.

When it happens

Trigger: A query like @facets(not(a AND b)) constructed so `not` wraps multiple children, or an empty/invalid not expression reaching preprocessFilter.

Common situations: Hand-written queries with ambiguous nesting; generated queries from ORMs/tools that mis-parenthesize NOT; converting SQL NOT IN patterns to Dgraph facet syntax.

Related errors

AI-assisted analysis of dgraph-io/dgraph@759e242be6 (2026-09-01). Data as JSON: /api/errors/fbce83144d7af614. Report an issue: GitHub.