From 69b44c34872573f205d41570a49fb1a145562210 Mon Sep 17 00:00:00 2001
From: Jason Turim <jason@opscanvas.com>
Date: Thu, 9 Jan 2025 11:45:12 -0500
Subject: [PATCH] Include the field name in error messages when scanning
 structs

---
 pgtype/json_test.go  |  4 ++--
 pgtype/jsonb_test.go |  4 ++--
 pgtype/xml_test.go   |  2 +-
 query_test.go        |  2 +-
 rows.go              | 11 ++++++++---
 values_test.go       |  4 +++-
 6 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/pgtype/json_test.go b/pgtype/json_test.go
index 6277fc8bb..8bb6ff77b 100644
--- a/pgtype/json_test.go
+++ b/pgtype/json_test.go
@@ -191,11 +191,11 @@ func TestJSONCodecUnmarshalSQLNull(t *testing.T) {
 		// A string cannot scan a NULL.
 		str := "foobar"
 		err = conn.QueryRow(ctx, "select null::json").Scan(&str)
-		require.EqualError(t, err, "can't scan into dest[0]: cannot scan NULL into *string")
+		require.EqualError(t, err, "can't scan into dest[0] (col: json): cannot scan NULL into *string")
 
 		// A non-string cannot scan a NULL.
 		err = conn.QueryRow(ctx, "select null::json").Scan(&n)
-		require.EqualError(t, err, "can't scan into dest[0]: cannot scan NULL into *int")
+		require.EqualError(t, err, "can't scan into dest[0] (col: json): cannot scan NULL into *int")
 	})
 }
 
diff --git a/pgtype/jsonb_test.go b/pgtype/jsonb_test.go
index f34b3ad51..031cd3768 100644
--- a/pgtype/jsonb_test.go
+++ b/pgtype/jsonb_test.go
@@ -66,11 +66,11 @@ func TestJSONBCodecUnmarshalSQLNull(t *testing.T) {
 		// A string cannot scan a NULL.
 		str := "foobar"
 		err = conn.QueryRow(ctx, "select null::jsonb").Scan(&str)
-		require.EqualError(t, err, "can't scan into dest[0]: cannot scan NULL into *string")
+		require.EqualError(t, err, "can't scan into dest[0] (col: jsonb): cannot scan NULL into *string")
 
 		// A non-string cannot scan a NULL.
 		err = conn.QueryRow(ctx, "select null::jsonb").Scan(&n)
-		require.EqualError(t, err, "can't scan into dest[0]: cannot scan NULL into *int")
+		require.EqualError(t, err, "can't scan into dest[0] (col: jsonb): cannot scan NULL into *int")
 	})
 }
 
diff --git a/pgtype/xml_test.go b/pgtype/xml_test.go
index 0f755e96f..de7de4d20 100644
--- a/pgtype/xml_test.go
+++ b/pgtype/xml_test.go
@@ -79,7 +79,7 @@ func TestXMLCodecUnmarshalSQLNull(t *testing.T) {
 		// A string cannot scan a NULL.
 		str := "foobar"
 		err = conn.QueryRow(ctx, "select null::xml").Scan(&str)
-		assert.EqualError(t, err, "can't scan into dest[0]: cannot scan NULL into *string")
+		assert.EqualError(t, err, "can't scan into dest[0] (col: xml): cannot scan NULL into *string")
 	})
 }
 
diff --git a/query_test.go b/query_test.go
index a6a26ad77..2c8410248 100644
--- a/query_test.go
+++ b/query_test.go
@@ -420,7 +420,7 @@ func TestConnQueryReadWrongTypeError(t *testing.T) {
 		t.Fatal("Expected Rows to have an error after an improper read but it didn't")
 	}
 
-	if rows.Err().Error() != "can't scan into dest[0]: cannot scan int4 (OID 23) in binary format into *time.Time" {
+	if rows.Err().Error() != "can't scan into dest[0] (col: n): cannot scan int4 (OID 23) in binary format into *time.Time" {
 		t.Fatalf("Expected different Rows.Err(): %v", rows.Err())
 	}
 
diff --git a/rows.go b/rows.go
index f23625d4c..268559ce6 100644
--- a/rows.go
+++ b/rows.go
@@ -272,7 +272,7 @@ func (rows *baseRows) Scan(dest ...any) error {
 
 		err := rows.scanPlans[i].Scan(values[i], dst)
 		if err != nil {
-			err = ScanArgError{ColumnIndex: i, Err: err}
+			err = ScanArgError{ColumnIndex: i, FieldName: fieldDescriptions[i].Name, Err: err}
 			rows.fatal(err)
 			return err
 		}
@@ -334,11 +334,16 @@ func (rows *baseRows) Conn() *Conn {
 
 type ScanArgError struct {
 	ColumnIndex int
+	FieldName   string
 	Err         error
 }
 
 func (e ScanArgError) Error() string {
-	return fmt.Sprintf("can't scan into dest[%d]: %v", e.ColumnIndex, e.Err)
+	if e.FieldName == "?column?" { // Don't include the fieldname if it's unknown
+		return fmt.Sprintf("can't scan into dest[%d]: %v", e.ColumnIndex, e.Err)
+	}
+
+	return fmt.Sprintf("can't scan into dest[%d] (col: %s): %v", e.ColumnIndex, e.FieldName, e.Err)
 }
 
 func (e ScanArgError) Unwrap() error {
@@ -366,7 +371,7 @@ func ScanRow(typeMap *pgtype.Map, fieldDescriptions []pgconn.FieldDescription, v
 
 		err := typeMap.Scan(fieldDescriptions[i].DataTypeOID, fieldDescriptions[i].Format, values[i], d)
 		if err != nil {
-			return ScanArgError{ColumnIndex: i, Err: err}
+			return ScanArgError{ColumnIndex: i, FieldName: fieldDescriptions[i].Name, Err: err}
 		}
 	}
 
diff --git a/values_test.go b/values_test.go
index 132b79a2a..4a5df8c65 100644
--- a/values_test.go
+++ b/values_test.go
@@ -3,6 +3,7 @@ package pgx_test
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"net"
 	"os"
 	"reflect"
@@ -215,7 +216,8 @@ func testJSONInt16ArrayFailureDueToOverflow(t *testing.T, conn *pgx.Conn, typena
 	input := []int{1, 2, 234432}
 	var output []int16
 	err := conn.QueryRow(context.Background(), "select $1::"+typename, input).Scan(&output)
-	if err == nil || err.Error() != "can't scan into dest[0]: json: cannot unmarshal number 234432 into Go value of type int16" {
+	expectedMessage := fmt.Sprintf("can't scan into dest[0] (col: %s): json: cannot unmarshal number 234432 into Go value of type int16", typename)
+	if err == nil || err.Error() != expectedMessage {
 		t.Errorf("%s: Expected *json.UnmarshalTypeError, but got %v", typename, err)
 	}
 }