Map response to a struct using Golang - go

Question

I am attempting to map a response from an API to a struct using Golang.
The JSON that comes back when I view the link in the browser is below:
{
"GBP": 657.54
}
And I just want to map it to a simple struct like so:
type Price struct {
Name string
Value float64
}
Here is my current code.
func FetchCoinPrice(fsym string, tsyms string) Price {
url := fmt.Sprintf("https://min-api.cryptocompare.com/data/price?fsym=" + fsym + "&tsyms=" + tsyms)
fmt.Println("Requesting data from " + url)
price := Price{}
// getting the data using http
request, err := http.Get(url)
if err != nil {
log.Fatal(err.Error())
}
// Read the response body using ioutil
body, err := ioutil.ReadAll(request.Body)
if err != nil {
log.Fatal(err.Error())
}
defer request.Body.Close()
if request.StatusCode == http.StatusOK {
json.Unmarshal(body, &price)
}
return price
}
At the moment all I receive is an empty struct, I know the link is bringing back the correct data and I've tested it in my browser.

Answer 1

The mapping doesn't work that way. Instead, you should use a map:
data := []byte(`{
"GBP": 657.54
}`)
priceMap := map[string]float64{}
err := json.Unmarshal(data, &priceMap)
// Check your errors!
if err != nil {
log.Fatal(err.Error())
}
fmt.Println(priceMap)
This will print:
map[GBP:657.54]
You can then iterate over the map and build the struct you mentioned above, or just access the entry directly if you know the currency. eg: priceMap["GBP"]
You should really check your errors, especially if you're not getting the output you expect from Unmarshal.

Answer 2

The problem is that the unmarshaler cannot guess that keys in a JSON object should correspond to some value in a struct. Golang JSON mapping simply doesn't work that way.
However, you can make your "Price" type implement json.Unmarshaler to deserialize a message into a map of floats (map[string]float64) then ensure the shape is right and populate the struct accordingly:
func (p *Price) UnmarshalJSON(bs []byte) error {
kvs := map[string]float64{}
err := json.Unmarshal(bs, &kvs)
if err != nil {
return err
}
if len(kvs) != 1 {
return fmt.Errorf("expected 1 key, got %d", len(kvs))
}
for name, value := range kvs {
p.Name, p.Value = name, value
}
return nil
}
func main() {
jsonstr := `[{"GBP":657.54},{"USD":123.45}]`
ps := []Price{}
err := json.Unmarshal([]byte(jsonstr), &ps)
if err != nil {
panic(err)
}
// ps=[]main.Price{
// main.Price{Name:"GBP", Value:657.54},
// main.Price{Name:"USD", Value:123.45}
// }
}