JSON unmarshal integer field into a string

Question

I am struggling with deserializing a integer into a string struct field. The struct field is a string and is expected to be assignable from users of my library. That\'s why I wa

Answer 1

You can customize how the data structure is unamrshaled by implementing the json.Unamrshaler interface.

The simplest way to handle unknown types is to nnmarshal the JSON into an intermediate structure, and handle the type assertions and validation during deserialization:

type test struct {
    Foo string `json:"foo"`
}

func (t *test) UnmarshalJSON(d []byte) error {
    tmp := struct {
        Foo interface{} `json:"foo"`
    }{}

    if err := json.Unmarshal(d, &tmp); err != nil {
        return err
    }

    switch v := tmp.Foo.(type) {
    case float64:
        t.Foo = strconv.Itoa(int(v))
    case string:
        t.Foo = v
    default:
        return fmt.Errorf("invalid value for Foo: %v", v)
    }

    return nil
}

https://play.golang.org/p/t0eI4wCxdB

Answer 2

To handle big structs you can use embedding.

Updated to not discard possibly previously set field values.

func (t *T) UnmarshalJSON(d []byte) error {
    type T2 T // create new type with same structure as T but without its method set!
    x := struct{
        T2 // embed
        Foo json.Number `json:"foo"`
    }{T2: T2(*t)} // don't forget this, if you do and 't' already has some fields set you would lose them

    if err := json.Unmarshal(d, &x); err != nil {
        return err
    }
    *t = T(x.T2)
    t.Foo = x.Foo.String()
    return nil
}

https://play.golang.org/p/BytXCeHMvt