From edfdaf15c673e383f5892ab484e0d4201c26adc6 Mon Sep 17 00:00:00 2001
From: Jack Christensen <jack@jackchristensen.com>
Date: Fri, 20 Nov 2015 14:02:49 -0600
Subject: [PATCH] Rows.Scan errors now include which argument caused error

---
 CHANGELOG.md   |  4 ++++
 query.go       | 19 ++++++++++++++-----
 query_test.go  |  4 ++++
 values_test.go |  3 +--
 4 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 42a15fa2..14393272 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# Tip
+
+* Rows.Scan errors now include which argument caused error
+
 # 2.7.1 (October 26, 2015)
 
 * Disable SSL renegotiation
diff --git a/query.go b/query.go
index 5871d628..8c0e9d07 100644
--- a/query.go
+++ b/query.go
@@ -214,6 +214,15 @@ func (rows *Rows) nextColumn() (*ValueReader, bool) {
 	return &rows.vr, true
 }
 
+type scanArgError struct {
+	col int
+	err error
+}
+
+func (e scanArgError) Error() string {
+	return fmt.Sprintf("can't scan into dest[%d]: %v", e.col, e.err)
+}
+
 // Scan reads the values from the current row into dest values positionally.
 // dest can include pointers to core types, values implementing the Scanner
 // interface, and []byte. []byte will skip the decoding process and directly
@@ -225,7 +234,7 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 		return err
 	}
 
-	for _, d := range dest {
+	for i, d := range dest {
 		vr, _ := rows.nextColumn()
 
 		// Check for []byte first as we allow sidestepping the decoding process and retrieving the raw bytes
@@ -244,7 +253,7 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 		} else if s, ok := d.(Scanner); ok {
 			err = s.Scan(vr)
 			if err != nil {
-				rows.Fatal(err)
+				rows.Fatal(scanArgError{col: i, err: err})
 			}
 		} else if vr.Type().DataType == JsonOid || vr.Type().DataType == JsonbOid {
 			decodeJson(vr, &d)
@@ -292,7 +301,7 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 				case TimestampOid:
 					*v = decodeTimestamp(vr)
 				default:
-					rows.Fatal(fmt.Errorf("Can't convert OID %v to time.Time", vr.Type().DataType))
+					rows.Fatal(scanArgError{col: i, err: fmt.Errorf("Can't convert OID %v to time.Time", vr.Type().DataType)})
 				}
 			case *net.IPNet:
 				*v = decodeInet(vr)
@@ -319,12 +328,12 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 						}
 					}
 				}
-				rows.Fatal(fmt.Errorf("Scan cannot decode into %T", d))
+				rows.Fatal(scanArgError{col: i, err: fmt.Errorf("Scan cannot decode into %T", d)})
 			}
 
 		}
 		if vr.Err() != nil {
-			rows.Fatal(vr.Err())
+			rows.Fatal(scanArgError{col: i, err: vr.Err()})
 		}
 
 		if rows.Err() != nil {
diff --git a/query_test.go b/query_test.go
index f6d969d5..2f9e7ee9 100644
--- a/query_test.go
+++ b/query_test.go
@@ -161,6 +161,10 @@ 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]: Can't convert OID 23 to time.Time" {
+		t.Fatalf("Expected different Rows.Err(): %v", rows.Err())
+	}
+
 	ensureConnValid(t, conn)
 }
 
diff --git a/values_test.go b/values_test.go
index a63a8cc9..8a54421e 100644
--- a/values_test.go
+++ b/values_test.go
@@ -1,7 +1,6 @@
 package pgx_test
 
 import (
-	"encoding/json"
 	"github.com/jackc/pgx"
 	"net"
 	"reflect"
@@ -196,7 +195,7 @@ func testJsonInt16ArrayFailureDueToOverflow(t *testing.T, conn *pgx.Conn, typena
 	input := []int{1, 2, 234432}
 	var output []int16
 	err := conn.QueryRow("select $1::"+typename, input).Scan(&output)
-	if _, ok := err.(*json.UnmarshalTypeError); !ok {
+	if err.Error() != "can't scan into dest[0]: json: cannot unmarshal number 234432 into Go value of type int16" {
 		t.Errorf("%s: Expected *json.UnmarkalTypeError, but got %v", typename, err)
 	}
 }