Go のリフレクションを使用して JSON アンマーシャリングをカスタマイズするにはどうすればよいですか?

<code class="json">{
  "I": 3,
  "S": {
    "phone": {
      "sales": "2223334444"
    }
  }
}</code>

<code class="go">type A struct {
    I int64
    S string `sql:"type:json"`
}</code>

<code class="go">func main() {
    a := A{}

    // Unmarshal the JSON data into a byte slice
    var data []byte

    // Iterate over the fields of the struct
    typ := reflect.TypeOf(a)
    val := reflect.ValueOf(a)
    for i := 0; i < typ.NumField(); i++ {
        f := typ.Field(i)

        // Check if the field has a "json" tag
        if f.Tag.Get("json") == "" {
            continue
        }

        // Retrieve the field value
        fv := val.Field(i)

        // Unmarshall the JSON data into the field as a string
        if err := json.Unmarshal(data, &fv); err != nil {
            log.Fatal(err)
        }
    }

    fmt.Println(a)
}</code>

<code class="go">// RawString is a raw encoded JSON object.
// It implements Marshaler and Unmarshaler and can
// be used to delay JSON decoding or precompute a JSON encoding.
type RawString string

// MarshalJSON returns *m as the JSON encoding of m.
func (m *RawString) MarshalJSON() ([]byte, error) {
    return []byte(*m), nil
}

// UnmarshalJSON sets *m to a copy of data.
func (m *RawString) UnmarshalJSON(data []byte) error {
    if m == nil {
        return errors.New("RawString: UnmarshalJSON on nil pointer")
    }
    *m += RawString(data)
    return nil
}

const data = `{"i":3, "S":{"phone": {"sales": "2223334444"}}}`

type A struct {
    I int64
    S RawString `sql:"type:json"`
}

func main() {
    a := A{}
    err := json.Unmarshal([]byte(data), &a)
    if err != nil {
        log.Fatal("Unmarshal failed", err)
    }
    fmt.Println("Done", a)
}</code>