Type checking json values when unmarshaling in Go

user_14237865 0
Viewed 135

I'm working with an API that usually provides json string values but they will sometimes provide numbers instead. For example, 99% of the time it's like this:

{
    "Description": "Doorknob",
    "Amount": "3.25"
}

But for whatever reason, sometimes it's like this:

{
    "Description": "Lightbulb",
    "Amount": 4.70
}

Here is the struct I'm using which works in 99% of the time:

type Cart struct {
    Description string `json:"Description"`
    Amount      string `json:"Amount"`
}

But it'll break when they provide a numeric amount. What's the best way to type check when unmarshaling a struct?

playground: https://play.golang.org/p/S_gp2sQC5-A

go json
2 Answers

For the general case you may use interface{} as described in Burak Serdar's answer.

For numbers specifically, there's the json.Number type: it accepts both JSON numbers and JSON strings, and it can "automatically" parse the number if it's given as string with Number.Int64() or Number.Float64(). No custom marshalers / unmarshalers are required.

type Cart struct {
    Description string      `json:"Description"`
    Amount      json.Number `json:"Amount"`
}

Testing it:

var (
    cart1 = []byte(`{
    "Description": "Doorknob",
    "Amount": "3.25"
}`)

    cart2 = []byte(`{
    "Description": "Lightbulb",
    "Amount": 4.70
}`)
)

func main() {
    var c1, c2 Cart
    if err := json.Unmarshal(cart1, &c1); err != nil {
        panic(err)
    }
    fmt.Printf("%+v\n", c1)
    if err := json.Unmarshal(cart2, &c2); err != nil {
        panic(err)
    }
    fmt.Printf("%+v\n", c2)
}

Output (try it on the Go Playground):

{Description:Doorknob Amount:3.25}
{Description:Lightbulb Amount:4.70}
user_17055980

If that field can be a string or int, you can use an interface{}, and then figure out the underlying value:

type Cart struct {
    Description string `json:"Description"`
    Amount      interface{} `json:"Amount"`
}

func (c Cart) GetAmount() (float64,error) {
  if d, ok:=c.Amount.(float64); ok {
     return d,nil
  }
  if s, ok:=c.Amount.(string); ok {
     return strconv.ParseFloat(s,64)
  }
  return 0, errors.New("Invalid amount")
}
user_119239990
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