How to Handle Nested JSON Structures and Avoid \'cannot unmarshal string into Go struct field\' Errors?

Unmarshaling JSON with Nested Structures

When working with complex JSON responses, occasionally you may encounter errors like "cannot unmarshal string into Go struct field". This typically occurs when the JSON response contains a string value that represents another JSON structure.

Consider this incomplete Go struct ManifestResponse and a corresponding JSON response:

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"`
}

{
    "name": "library/redis",
    "tag": "latest",
    "architecture": "amd64",
    "history": [
        {
            "v1Compatibility": "{\"id\":\"ef8a93741134ad37c834c32836aefbd455ad4aa4d1b6a6402e4186dfc1feeb88\",\"parent\":\"9c8b347e3807201285053a5413109b4235cca7d0b35e7e6b36554995cfd59820\",\"created\":\"2017-10-10T02:53:19.011435683Z\"}"
        }
    ]
}

When attempting to unmarshal the JSON into the Go struct, you might encounter 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\"" }

The issue stems from the fact that v1Compatibility in the JSON response is a string value containing JSON content. To address this, we can implement a two-pass unmarshaling:

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"`
}

Then perform the following unmarshaling process:

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
}

This approach allows you to handle nested JSON structures with a two-step unmarshaling process, preventing the "cannot unmarshal string into Go struct field" error.

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