A field in JSON can come in with different types, can I represent it with a single struct in Go?

Question

An API endpoint returns a set of posts, and a field attached_image in each post can come in as one of two types

JSON snippet:

{
   "attached_image": {   // Empty string if there is no image
      "url": "https://cataas.com/c?width=500&height=500",
      "width": 500,
      "height": 600
   },
}

Is there any way to implement it, so that if attached_image is an empty string, the field would be a string, and otherwise it would become a struct, without checking the type of the value that comes from json.Unmarshal?

why empty string? if there is no image write format will be `null` right ? — Eklavya, Jun 06 '20 at 19:11
Your JSON cannot be unmarshaled into a struct, it would be a map right? — Inian, Jun 06 '20 at 19:12
@Eklavya that is decision taken by the API creators, I cannot change that — frogstair, Jun 06 '20 at 19:16
@Inian yes, it would be a map, but I would want a struct, and I could of course compare it and assign each value in the map to the struct, but I feel like there is a better way to do that — frogstair, Jun 06 '20 at 19:20
Write as custom unmarshaler for your struct then where you handle those case — Eklavya, Jun 06 '20 at 19:26

thwd · Accepted Answer · 2020-06-06T19:41:49.420

Declare a type to represent the image. Implement the json.Unmarshaler interface on the type. Handle the case where the image JSON is "" or an object in that implemention.

type Image struct {
    URL    string
    Width  int
    Height int
}

func (i *Image) UnmarshalJSON(data []byte) error {
    // Do nothing if data is the empty string.
    if bytes.Equal(data, []byte(`""`)) {
        return nil
    }

    // type imageX has same underlying struct as Image,
    // but does not have any methods. This prevents
    // json.Unmarshal from recursively calling the present
    // method.
    type imageX Image

    return json.Unmarshal(data, (*imageX)(i))
}

Use it like this:

type Result struct {
    AttachedImage *Image `json:"attached_image"`
}

var r Result
err := json.Unmarshal(data, &r)

Run it on the playground.

A field in JSON can come in with different types, can I represent it with a single struct in Go?

1 Answers1