json：无法将对象解组为Auction.Item类型的Go值

Question

I have a problem deserializing my object. I use an interface to this object to call the Serialization, and from reading the output the serialization works perfectly. Here is the underlying struct of my object:

type pimp struct {
    Price       int
    ExpDate     int64
    BidItem     Item
    CurrentBid  int
    PrevBidders []string
}

And here's the interface it implements:

type Pimp interface {
    GetStartingPrice() int
    GetTimeLeft() int64
    GetItem() Item
    GetCurrentBid() int
    SetCurrentBid(int)
    GetPrevBidders() []string
    AddBidder(string) error
    Serialize() ([]byte, error)
}

The Serialize() method:

func (p *pimp) Serialize() ([]byte, error) {
    return json.Marshal(*p)
}

As you may have noticed, pimp has a variable by the name of Item. This item is also, an interface:

type item struct {
    Name string
}

type Item interface {
    GetName() string
}

Now serializing a sample of such an object returns the following JSON:

{"Price":100,"ExpDate":1472571329,"BidItem":{"Name":"ExampleItem"},"CurrentBid":100,"PrevBidders":[]}

Here is my deserialization code:

func PimpFromJSON(content []byte) (Pimp, error) {
    p := new(pimp)
    err := json.Unmarshal(content, p)
    return p, err
}

Running it, however, gives me the following error:

json: cannot unmarshal object into Go value of type Auction.Item

Any help is appreciated.

Answer 1

The unmarshaler does not know the concrete type to use for the nil BidItem field. You can fix this by setting the field to a value of the appropriate type:

func PimpFromJSON(content []byte) (Pimp, error) {
    p := new(pimp)
    p.BidItem = &item{}
    err := json.Unmarshal(content, p)
    return p, err
}

playground example