填充作为参数指定的接口类型的一部分（例如，为数据库/ sql实现ScanAll）

Question

How could we implement a function which would return all rows resulting from a SQL query and convert them to dest which is an interface array (may not work as Scan) ?

I assume the destination array would have to be given as an argument to the function. But then, I still do not know how I am supposed to complete the implementation:

func GetAll(query string, dest interface{}) error {

  rows, err := s.db.Query(query)
  if err != nil {
    return err
  }
  defer rows.Close()

  for rows.Next() {
    var destRow ??? /* do not have a type. using reflect.TypeOf(dest).Elem()? */
    err := rows.Scan(&destRow)
    if err != nil {
      return err
    }
    dest = append(dest, destRow) /* would even compile? */
  }
  return nil
}

It does not look like much different from what json.Unmarshal have to do actually...

Answer 1

Here's a solution using the reflect package. Call the function with a pointer to a slice.

func GetAll(query string, dest interface{}) error {

    // Return error if dest is not a pointer to a slice.
    slice := reflect.ValueOf(dest)
    if slice.Kind() != reflect.Ptr {
        return errors.New("dest must be pointer")
    }
    slice = slice.Elem()
    if slice.Kind() != reflect.Slice {
        return errors.New("dest must be pointer to struct")
    }

    rows, err := db.Query(query)
    if err != nil {
        return err
    }
    defer rows.Close()

    for rows.Next() {
        // Create a new slice element. The variable elementp holds a 
        // pointer to the new element.
        elementp := reflect.New(slice.Type().Elem())

        err := rows.Scan(elementp.Interface())
        if err != nil {
            return err
        }

        // Append the element to the slice.
        slice.Set(reflect.Append(slice, elementp.Elem()))
    }
    return nil
}

The above code works with standard database/sql package when the query result has a single column and dest is a pointer to a slice of a type appropriate for that column. Here's an example:

var names []string
if err := GetAll(db, "select name from people", &names); err != nil {
    // handle error
}

This code should also work with a database package that supports scanning multiple columns to a struct.