Extension for sql package

sql.go

// Fixing some of the errors in the:
// https://gist.github.com/PumpkinSeed/b4993c6ad20ea90e3da8c991a90a91e1
//
// 1. It was only able to extract database information, based upon a Struct.
// The code now can deal with a struct or a slice with structs.
// 
// 2. The code relied on the json tag in the struct.
// You need to match the data with database fields, not the output fields. 
// This will match up more 1 to 1 as you are in controle of your Database naming and selecting of that data.
// If a client expect different named exported json fields, ... 
//
// 3. The code while designed for a single struct, kept looping over all the rows. 
// So if you tried to extract one result and did not use a limitor ( LIMIT ) in your SQL code, it will have looped the column information ..N times
// 
// 4. Some data types like Int had been missing
//
// 5. Fix a few bugs


type User struct {
	ID        int    `db:"id"`
	Firstname string `db:"firstname" json:"first_name"`
	Lastname  string `db:"lastname"  json:"last_name"`
	Age       int    `db:"age"       json:"age,omitempty"`
}

// QueryStruct allows a Struct or Slice with Struct as result
//
// Example:
//
// contacts := []Contact{}
// QueryStruct(&contacts, "SELECT * FROM contact")
//
// contact := Contact{}
// QueryOne(&contact, "SELECT * FROM contact")
//
func (connection *Connection) QueryStruct(dest interface{}, query string, args ...interface{}) bool {

	rows, err := connection.sql.Query(query, args...)
	if err != nil {
		log.Println("DB: QueryAll failed: " + err.Error())
		return false
	}
	defer rows.Close()

	err = structScan(rows, dest)
	if err != nil {
		log.Println("DB: QueryAll structScan failed: " + err.Error())
		return false
	}

	return true
}

// structScan will scan the results of a database query and try to merge then with the supplied struct
//
// Example: []Users, User
//
// Are both valid structs as the routing will automatically figure out if the results are a slice with struct or a pure struct
func structScan(rows *sql.Rows, model interface{}) error {

	v := reflect.ValueOf(model)
	if v.Kind() != reflect.Ptr {
		return errors.New("must pass a pointer, not a value, to StructScan destination") // @todo add new error message
	}

	v = reflect.Indirect(v)
	t := v.Type()

	// Find out if what we are using is a Struct ( Query One ) or a Slice with Structs ( QueryAll )
	isStruct, isSlice := false, false
	if t.Kind() == reflect.Slice {
		isSlice = true
	} else if t.Kind() == reflect.Struct {
		isStruct = true
	}

	// Ensure we only get the column information one time!
	cols, _ := rows.Columns()
	columns := make([]interface{}, len(cols))
	columnPointers := make([]interface{}, len(cols))
	for i := range columns {
		columnPointers[i] = &columns[i]
	}

	var m []map[string]interface{}
	for rows.Next() {
		if err := rows.Scan(columnPointers...); err != nil {
			return err
		}

		x := make(map[string]interface{})
		for i, colName := range cols {
			val := columnPointers[i].(*interface{})
			x[colName] = *val
		}
		m = append(m, x)

		// If  we are dealing with a struct. There is no point in looping over all the results, if they are more then one!
		if isStruct == true {
			break
		}
	}

	if isStruct == true {
		if len(m) > 0 { // Ensure we have data fields!
			changeStruct(v, t, m[0])
		}
	}

	if isSlice == true {
		if len(m) > 0 { // Ensure we have data in the slice!
			var elem reflect.Value
			for _, d := range m {

				typ := v.Type().Elem()
				elem = reflect.New(typ).Elem()

				changeStruct(elem, typ, d)
				v.Set(reflect.Append(v, elem))
			}
		}
	}

	return nil
}

func changeStruct(v reflect.Value, t reflect.Type, m map[string]interface{}) {
	for i := 0; i < v.NumField(); i++ {
		field := strings.Split(t.Field(i).Tag.Get("db"), ",")[0]

		if item, ok := m[field]; ok {
			if v.Field(i).CanSet() {
				if item != nil {
					switch v.Field(i).Kind() {
					case reflect.String:
						v.Field(i).SetString(item.(string)) //	s := bytesToString(item.([]uint8))
					case reflect.Float32, reflect.Float64:
						v.Field(i).SetFloat(item.(float64))
					case reflect.Int, reflect.Int32, reflect.Int64:
						v.Field(i).SetInt(item.(int64))
					case reflect.Bool:
						v.Field(i).Set(reflect.ValueOf(!(item.(int64) == 0)))
					case reflect.Ptr:
						if reflect.ValueOf(item).Kind() == reflect.Bool {
							itemBool := item.(bool)
							v.Field(i).Set(reflect.ValueOf(&itemBool))
						}
					case reflect.Struct:
						v.Field(i).Set(reflect.ValueOf(item))
					default:
						fmt.Println(t.Field(i).Name, ": ", v.Field(i).Kind(), " - > - ", reflect.ValueOf(item).Kind()) // @todo remove after test out the Get methods
					}
				}
			}
		}
	}
}

I was having issues with MySQL representing ID as uint8. So i modified the script and came up with this.

func changeStruct(v reflect.Value, t reflect.Type, m map[string]interface{}) {

	for i := 0; i < v.NumField(); i++ {

		field := strings.Split(t.Field(i).Tag.Get("json"), ",")[0]
		item, ok := m[field]

		if ok {
			if v.Field(i).CanSet() {
				if item != nil {

					switch v.Field(i).Kind() {

					case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
						uinttext := (item.(interface{}).([]uint8))
						if intVal, err := strconv.ParseUint(string(uinttext), 10, 64); err == nil {
							v.Field(i).SetUint(uint64(intVal))
						}

					case reflect.String:
						v.Field(i).SetString(fmt.Sprintf("%s", item))

					case reflect.Float32, reflect.Float64:
						v.Field(i).SetFloat(item.(float64))

					case reflect.Int, reflect.Int32, reflect.Int64:
						v.Field(i).SetInt(item.(int64))

					case reflect.Bool:
						v.Field(i).Set(reflect.ValueOf(!(item.(int64) == 0)))

					case reflect.Ptr:
						if reflect.ValueOf(item).Kind() == reflect.Bool {
							itemBool := item.(bool)
							v.Field(i).Set(reflect.ValueOf(&itemBool))
						}

					case reflect.Struct:
						v.Field(i).Set(reflect.ValueOf(item))

					default:
						
					}
				}
			}
		}
	}
}

Thanks for showing me the way to fix this.