MarshalJSON error , invalid character "g" after top-level

Question

I made a custom type for my IDs:

type ID uint

func (id ID) MarshalJSON() ([]byte, error) {
    e, _ := HashIDs.Encode([]int{int(id)})
    fmt.Println(e) /// 34gj
    return []byte(e), nil
}

func (id *ID) Scan(value interface{}) error {
    *id = ID(value.(int64))
    return nil
}

I use the HashIDs package to encode my ids so that user wont be able to read them on client side. But I'm getting this error:

json: error calling MarshalJSON for type types.ID: invalid character 'g' after top-level value

Solution

34gj is not a valid JSON and hence not a valid string representation of your ID. You probably want to wrap this with double quotation mark to indicate this is a string, i.e. returning "34gj".

Try:

func (id ID) MarshalJSON() ([]byte, error) {
    e, _ := HashIDs.Encode([]int{int(id)})
    fmt.Println(e) /// 34gj
    return []byte(`"` + e + `"`), nil
}

http://play.golang.org/p/0ESimzPbAx

Instead of doing it by hand you can also call marshaller for the string, by simply replacing your return with return json.Marshal(e).

My guess would be that invalid character 'g' in your error is due to initial part of the value is being treated as a number and then unexpected character occurs.

Comments

[Emad Ghasemi]

Yes, it might... I had tought the marshaller wrap it it self in " " but don't know why this happend... anyway this solved it, thanks

[Emad Ghasemi]

I hope there's a better explanation for this behavior

[tomasz]

Emad, it can't wrap it for you with " automatically as by implementing marshaller you're supposed to provide a valid raw JSON. So you can, for example, return {"id": "abc"} (not a string!). Obviously wrapping it with quotes would be wrong. You can always use return json.Marshal(e) in your code to provide a valid string representation without worrying about quotes (added this comment to my answer).