Decoding JSON int into string

Question

I have this simple JSON string where I want user_id to be converted into string when doing json.Unmarshal:

{"user_id": 344, "user_name": "shiki"}

I have tried this:

type User struct {
  Id       string `json:"user_id,int"`
  Username string `json:"user_name"`
}

func main() {
  input := `{"user_id": 344, "user_name": "shiki"}`
  user := User{}
  err := json.Unmarshal([]byte(input), &user)
  if err != nil {
    panic(err)
  }

  fmt.Println(user)
}

But I just get this error:

panic: json: cannot unmarshal number into Go value of type string

Playground link: http://play.golang.org/p/mAhKYiPDt0

Dave C · Accepted Answer · 2020-02-23 14:33:03Z

73

You can use the type json.Number which is implemented as a string:

type User struct {
        Id       json.Number `json:"user_id"`
        Username string      `json:"user_name"`
}

Then you can simply convert it in any other code:

stringNumber := string(userInstance.Id)

Playground: https://play.golang.org/p/2BTtWKkt8ai

edited Feb 23, 2020 at 14:33

Dave C

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answered Jun 29, 2014 at 22:41

Simon Whitehead

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9 Comments

Shiki Over a year ago

True. I am currently going with that option (using just int). I am however asking if there's a way to automatically convert it during decoding. It'll be very convenient.

Simon Whitehead Over a year ago

Sorry - I am yet to write my own decoder (still new to Go). That said, it shouldn't be too hard. Googling custom unmarshal/decoder seems to throw up a few links.

eduncan911 Over a year ago

OMG. banging head against wall, table, threw laptop out window I've been searching for why I can't convert a json longitude into float64 for hours (days?) cause the source uses empty "" strings when there was none (bad source). So, it was up to me to make exceptional parsing for it. This was insane. "cannot unmarshal number into string", "cannot unmarshal string into float64", "cannot unmarshal X into XX". There is a "json.Number"?!?! ARgh. IT WORKED! +10000

STEEL Over a year ago

using json.Number works in sending POST data via http. But inside internal Structs I had to convert 8.0 into "8" is this okay ?

jspri Over a year ago

Is there any documentation on the ,Number annotation?

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