cannot unmarshal string into Go struct field

Question

I have the following (incomplete type) (which is a response from docker API from manifests endpoint v2 schema 1 https://docs.docker.com/registry/spec/manifest-v2-1/)

Answer 1

The problem is that v1Compatibility is a string value in the JSON; the value happens to be JSON content, but it's inside a JSON string, so can't be unmarshalled all in one step. You could instead unmarshal it in two passes:

type ManifestResponse struct {
    Name         string `json:"name"`
    Tag          string `json:"tag"`
    Architecture string `json:"architecture"`

    FsLayers []struct {
        BlobSum string `json:"blobSum"`
    } `json:"fsLayers"`

    History []struct {
        V1CompatibilityRaw string `json:"v1Compatibility"`
        V1Compatibility V1Compatibility
    } `json:"history"`
}

type V1Compatibility struct {
    ID              string `json:"id"`
    Parent          string `json:"parent"`
    Created         string `json:"created"`
}

And then:

var jsonManResp ManifestResponse
if err := json.Unmarshal([]byte(exemplar), &jsonManResp); err != nil {
    log.Fatal(err)
}
for i := range jsonManResp.History {
    var comp V1Compatibility
    if err := json.Unmarshal([]byte(jsonManResp.History[i].V1CompatibilityRaw), &comp); err != nil {
        log.Fatal(err)
    }
    jsonManResp.History[i].V1Compatibility = comp
}

Working playground example here: https://play.golang.org/p/QNsu5_63E0