Why is my Go struct unable to unmarshal a JSON string into a nested struct field, and how can I fix it using a two-pass unmarshalling approach?

Issue: Unable to Unmarshal String into Go Struct Field

In the context of handling a JSON response from the Docker API's /manifests endpoint (v2 schema 1), an error is encountered while attempting to deserialize it into a Go struct. 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"" }", indicates that the field in question is expected to be of a different type than the provided JSON data.

The following code snippet represents the problematic section:

type ManifestResponse struct {
    // ... other fields
    History []struct {
        V1Compatibility struct {
            ID              string `json:"id"`
            Parent          string `json:"parent"`
            Created         string `json:"created"`
        } `json:"v1Compatibility"`
    } `json:"history"`
}

// ...
if err = json.NewDecoder(res.Body).Decode(&jsonManResp); err != nil {
    log.Fatal(err)
}

The issue arises because the field V1Compatibility is a string in the JSON response. However, Golang expects it to be a struct of type V1Compatibility. To resolve this, a two-pass unmarshalling approach is necessary.

The revised code below demonstrates the workaround:

type ManifestResponse struct {
    // ... other fields
    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"`
}

// ...
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
}

In this solution, the V1CompatibilityRaw field is introduced to accommodate the string value from the JSON response. The second pass of unmarshalling populates the V1Compatibility field with the actual struct data.

This approach enables successful deserialization of the JSON response into the desired Go struct.

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