cannot unmarshal string into Go struct field - json

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/)
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 {
V1Compatibility struct {
ID string `json:"id"`
Parent string `json:"parent"`
Created string `json:"created"`
} `json:"v1Compatibility"`
} `json:"history"`
}
While getting the following response:
{ "schemaVersion": 1,
"name": "library/redis",
"tag": "latest",
"architecture": "amd64",
"history": [
{
"v1Compatibility": "{\"id\":\"ef8a93741134ad37c834c32836aefbd455ad4aa4d1b6a6402e4186dfc1feeb88\",\"parent\":\"9c8b347e3807201285053a5413109b4235cca7d0b35e7e6b36554995cfd59820\",\"created\":\"2017-10-10T02:53:19.011435683Z\"}"
}
]
}
While trying to deserialize it using the following snippet of code:
var jsonManResp ManifestResponse
if err = json.NewDecoder(res.Body).Decode(&jsonManResp); err != nil {
log.Fatal(err)
}
I get the following error:
json: cannot unmarshal string into Go struct field .v1Compatibility of type struct { ID string "json:\"id\""; Parent string "json:\"parent\""; Created string "json:\"created\"" }
Full playground code: https://play.golang.org/p/tHzf9GphWX
What might be the problem?

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