Unmarshal Json data in a specific struct

Question

I want to unmarshal the following JSON data in Go:

b := []byte(`{"Asks": [[21, 1], [22, 1]] ,"Bids": [[20, 1], [19, 1]]}`)

I know how to do that, i define a struct like this:

type Message struct {
    Asks [][]float64 `json:"Bids"`
    Bids [][]float64 `json:"Asks"`
}

What i don't know is if there is a simple way to specialize this a bit more. I would like to have the data after the unmarshaling in a format like this:

type Message struct {
    Asks []Order `json:"Bids"`
    Bids []Order `json:"Asks"`
}

type Order struct {
    Price float64
    Volume float64
}

So that i can use it later after unmarshaling like this:

m := new(Message)
err := json.Unmarshal(b, &m)
fmt.Println(m.Asks[0].Price)

I don't really know how to easy or idiomatically do that in GO so I hope that there is a nice solution for that.

Solution

You can do this with by implementing the json.Unmarshaler interface on your Order struct. Something like this should do:

func (o *Order) UnmarshalJSON(data []byte) error {
    var v [2]float64
    if err := json.Unmarshal(data, &v); err != nil {
        return err
    }
    o.Price = v[0]
    o.Volume = v[1]
    return nil
}

This basically says that the Order type should be decoded from a 2 element array of floats rather than the default representation for a struct (an object).

You can play around with this example here: http://play.golang.org/p/B35Of8H1e6