Fix scanning identical column names occurring >2 times (#2080)

Fix the indexing logic used in selectedColumnsMap to skip fields
that have already been seen. The values of selectedColumns map
must be indexed relative to fields, not relative to selectFields.

main_test.go

@@ -581,6 +581,60 @@ func TestJoins(t *testing.T) {
 	}
 }
 
+type JoinedIds struct {
+	UserID           int64 `gorm:"column:id"`
+	BillingAddressID int64 `gorm:"column:id"`
+	EmailID          int64 `gorm:"column:id"`
+}
+
+func TestScanIdenticalColumnNames(t *testing.T) {
+	var user = User{
+		Name:  "joinsIds",
+		Email: "joinIds@example.com",
+		BillingAddress: Address{
+			Address1: "One Park Place",
+		},
+		Emails: []Email{{Email: "join1@example.com"}, {Email: "join2@example.com"}},
+	}
+	DB.Save(&user)
+
+	var users []JoinedIds
+	DB.Select("users.id, addresses.id, emails.id").Table("users").
+		Joins("left join addresses on users.billing_address_id = addresses.id").
+		Joins("left join emails on emails.user_id = users.id").
+		Where("name = ?", "joinsIds").Scan(&users)
+
+	if len(users) != 2 {
+		t.Fatal("should find two rows using left join")
+	}
+
+	if user.Id != users[0].UserID {
+		t.Errorf("Expected result row to contain UserID %d, but got %d", user.Id, users[0].UserID)
+	}
+	if user.Id != users[1].UserID {
+		t.Errorf("Expected result row to contain UserID %d, but got %d", user.Id, users[1].UserID)
+	}
+
+	if user.BillingAddressID.Int64 != users[0].BillingAddressID {
+		t.Errorf("Expected result row to contain BillingAddressID %d, but got %d", user.BillingAddressID.Int64, users[0].BillingAddressID)
+	}
+	if user.BillingAddressID.Int64 != users[1].BillingAddressID {
+		t.Errorf("Expected result row to contain BillingAddressID %d, but got %d", user.BillingAddressID.Int64, users[0].BillingAddressID)
+	}
+
+	if users[0].EmailID == users[1].EmailID {
+		t.Errorf("Email ids should be unique. Got %d and %d", users[0].EmailID, users[1].EmailID)
+	}
+
+	if int64(user.Emails[0].Id) != users[0].EmailID && int64(user.Emails[1].Id) != users[0].EmailID {
+		t.Errorf("Expected result row ID to be either %d or %d, but was %d", user.Emails[0].Id, user.Emails[1].Id, users[0].EmailID)
+	}
+
+	if int64(user.Emails[0].Id) != users[1].EmailID && int64(user.Emails[1].Id) != users[1].EmailID {
+		t.Errorf("Expected result row ID to be either %d or %d, but was %d", user.Emails[0].Id, user.Emails[1].Id, users[1].EmailID)
+	}
+}
+
 func TestJoinsWithSelect(t *testing.T) {
 	type result struct {
 		Name  string

scope.go

@@ -486,8 +486,10 @@ func (scope *Scope) scan(rows *sql.Rows, columns []string, fields []*Field) {
 		values[index] = &ignored
 
 		selectFields = fields
+		offset := 0
 		if idx, ok := selectedColumnsMap[column]; ok {
-			selectFields = selectFields[idx+1:]
+			offset = idx + 1
+			selectFields = selectFields[offset:]
 		}
 
 		for fieldIndex, field := range selectFields {
@@ -501,7 +503,7 @@ func (scope *Scope) scan(rows *sql.Rows, columns []string, fields []*Field) {
 					resetFields[index] = field
 				}
 
-				selectedColumnsMap[column] = fieldIndex
+				selectedColumnsMap[column] = offset + fieldIndex
 
 				if field.IsNormal {
 					break