How to Resolve \'json: cannot unmarshal string into Go struct field\' Error in Docker Manifest Decoding?

"cannot unmarshal string into Go struct field" Error in Manifest Decoding

While deserializing a JSON response from the Docker API, the 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""}" is encountered.

The issue stems from a mismatch between the response structure and the Go struct definition. Specifically, the "v1Compatibility" field in the JSON response is a string containing nested JSON content. The Go struct expects it to be a native Go struct, leading to the unmarshalling error.

To resolve this, a two-pass unmarshalling approach can be employed:

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

    FsLayers     []FSLayer `json:"fsLayers"`
    History       []HistEntry
}

type FSLayer struct {
    BlobSum string `json:"blobSum"`
}

type HistEntry struct {
    V1CompatibilityRaw string            `json:"v1Compatibility"`
    V1Compatibility   V1Compatibility   `json:"-"`
}

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

During the first pass, the JSON response is unmarshalled into the ManifestResponse struct with the V1CompatibilityRaw field populated:

var jsonManResp ManifestResponse
if err := json.Unmarshal([]byte(response), &jsonManResp); err != nil {
    log.Fatal(err)
}

In the second pass, each V1CompatibilityRaw string value is unmarshalled into the corresponding V1Compatibility struct:

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
}

By handling the nested JSON content in this manner, the error is resolved, and the correct data is unmarshalled into the Go struct.

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