From 90f0e64c53882440cd073a161306c0438bff0e19 Mon Sep 17 00:00:00 2001
From: SAY-5 <saiasish.cnp@gmail.com>
Date: Sun, 10 May 2026 16:26:18 -0700
Subject: [PATCH] fix(avro): error on complex (non-nullable) unions instead of
 silently dropping (#777)

Signed-off-by: SAY-5 <saiasish.cnp@gmail.com>
---
 arrow/avro/schema.go      | 11 ++++++++---
 arrow/avro/schema_test.go | 25 +++++++++++++++++++++++++
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/arrow/avro/schema.go b/arrow/avro/schema.go
index 4d9e76707..8eb4b91a3 100644
--- a/arrow/avro/schema.go
+++ b/arrow/avro/schema.go
@@ -139,13 +139,16 @@ func arrowSchemafromAvro(n *schemaNode) {
 		arrowSchemafromAvro(c)
 		n.arrowField = buildArrowField(n, arrow.MapOf(arrow.BinaryTypes.String, c.arrowField.Type), c.arrowField.Metadata)
 	case "union":
-		if n.schema.(*avro.UnionSchema).Nullable() {
-			if len(n.schema.(*avro.UnionSchema).Types()) > 1 {
-				n.schema = n.schema.(*avro.UnionSchema).Types()[1]
+		us := n.schema.(*avro.UnionSchema)
+		if us.Nullable() {
+			if len(us.Types()) > 1 {
+				n.schema = us.Types()[1]
 				n.union = true
 				n.nullable = true
 				arrowSchemafromAvro(n)
 			}
+		} else {
+			panic(fmt.Errorf("complex (non-nullable) avro union at '%v' is not supported", n.schemaPath()))
 		}
 	// Avro "fixed" field type = Arrow FixedSize Primitive BinaryType
 	case "fixed":
@@ -245,6 +248,8 @@ func iterateFields(n *schemaNode) {
 					c.nullable = true
 					arrowSchemafromAvro(c)
 				}
+			} else {
+				panic(fmt.Errorf("complex (non-nullable) avro union in field '%v' is not supported", f.Name()))
 			}
 		default:
 			n.schemaCache.Add(f.Name(), f.Type())
diff --git a/arrow/avro/schema_test.go b/arrow/avro/schema_test.go
index 33b6d2a05..b4b91b85a 100644
--- a/arrow/avro/schema_test.go
+++ b/arrow/avro/schema_test.go
@@ -18,10 +18,12 @@ package avro
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/apache/arrow-go/v18/arrow"
 	"github.com/apache/arrow-go/v18/arrow/avro/testdata"
+	avropkg "github.com/hamba/avro/v2"
 )
 
 func TestSchemaStringEqual(t *testing.T) {
@@ -174,3 +176,26 @@ func TestSchemaStringEqual(t *testing.T) {
 		})
 	}
 }
+
+func TestComplexUnionReportsError(t *testing.T) {
+	// Non-nullable union (e.g. [int, string]) is not supported and should
+	// produce a clear error rather than being silently dropped.
+	const avroSchemaJSON = `{
+		"type": "record",
+		"name": "WithComplexUnion",
+		"fields": [
+			{"name": "value", "type": ["int", "string"]}
+		]
+	}`
+	schema, err := avropkg.Parse(avroSchemaJSON)
+	if err != nil {
+		t.Fatalf("avro parse: %v", err)
+	}
+	got, err := ArrowSchemaFromAvro(schema)
+	if err == nil {
+		t.Fatalf("expected error for complex union, got schema=%v", got)
+	}
+	if !strings.Contains(err.Error(), "union") {
+		t.Fatalf("expected error to mention union, got: %v", err)
+	}
+}