How to Unmarshal Nested JSON within a String Field in Go?

Cannot Unmarshal String into Go Struct Field

In attempting to deserialize a Docker API response, the error "json: cannot unmarshal string into Go struct field .v1Compatibility" occurs. The JSON structure defines a v1Compatibility field as a string, but the actual response contains a JSON string within that field.

To resolve this issue, a two-pass unmarshaling approach is required:

1. Define a new field, V1CompatibilityRaw, in the Go struct to capture the raw string value.
2. Unmarshal the raw JSON string into a separate V1Compatibility struct.

Here's the modified Go struct:

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"`
}

After unmarshaling the raw JSON string, the V1Compatibility field can be updated with the parsed data:

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
}

This two-pass approach effectively handles the situation where a string field in the JSON response contains nested JSON content.

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