How to use rows.Scan of Go's database/sql - go

Question

I use database/sql and define a struct mapping to DB table columns(tag field):
// Users ...
type Users struct {
ID int64 `field:"id"`
Username string `field:"username"`
Password string `field:"password"`
Tel string `field:"tel"`
}
then I query:
rows, err := db.Query(sql) // select * from users
if err != nil {
fmt.Println(err)
}
defer rows.Close()
for rows.Next() {
user := new(Users)
// works but I don't think it is good code for too many columns
err = rows.Scan(&user.ID, &user.Username, &user.Password, &user.Tel)
// TODO: How to scan in a simple way
if err != nil {
fmt.Println(err)
}
fmt.Println("user: ", user)
list = append(list, *user)
}
if err := rows.Err(); err != nil {
fmt.Println(err)
}
As you can see for rows.Scan() , I have to write all columns , and I don't think it's a good way for 20 or more columns .
How to scan in a clear way.

Answer 1

It's a good practice for using reflect:
for rows.Next() {
user := Users{}
s := reflect.ValueOf(&user).Elem()
numCols := s.NumField()
columns := make([]interface{}, numCols)
for i := 0; i < numCols; i++ {
field := s.Field(i)
columns[i] = field.Addr().Interface()
}
err := rows.Scan(columns...)
if err != nil {
log.Fatal(err)
}
log.Println(user)
}

Answer 2

You may consider using jmoiron's sqlx package. It has support for assigning to a struct.
Excerpt from the readme:
type Place struct {
Country string
City sql.NullString
TelCode int
}
places := []Place{}
err = db.Select(&places, "SELECT * FROM place ORDER BY telcode ASC")
if err != nil {
fmt.Println(err)
return
}