How to Dynamically Add Fields to JSON Output from Unknown Go Structs?

Dynamically Enhancing JSON Output of Unknown Structs

Introduction
In Go, it is possible to enhance the JSON output of known structs by embedding them as anonymous fields. However, this technique only works with specific types. This article explores how to extend this functionality to unknown structs or interfaces.

Example
Consider the following example:

type example interface{}
type Data struct {
    Name string
}

func printInterface(val interface{}) {
    example1 := struct {
        example
        Extra string
    }{
        example: val,
        Extra: "text",
    }
    json.NewEncoder(os.Stdout).Encode(example1)
}

func main() {
    d := Data{Name: "name"}
    printInterface(&d)
}

This code prints the following JSON:

{"example":{"Name":"name"},"Extra":"text"}

Notice how the "Name" field is wrapped within an unnecessary "example" object.

Solution 1: Generating Dynamic Types with Reflection
To avoid the "example" object, we need to generate a dynamic type at runtime. We can use reflection to create a struct type that embeds the unknown interface and includes our additional "Extra" field.

func printInterface(val interface{}) {
    t2 := reflect.StructOf([]reflect.StructField{
        {
            Name:      "X",
            Anonymous: true,
            Type:      reflect.TypeOf(val),
        },
        {
            Name: "Extra",
            Type: reflect.TypeOf(""),
        },
    })

    v2 := reflect.New(t2).Elem()
    v2.Field(0).Set(reflect.ValueOf(val))
    v2.FieldByName("Extra").SetString("text")

    json.NewEncoder(os.Stdout).Encode(v2.Interface())
}

With this change, the output becomes:

{"Name":"name","Extra":"text"}

Solution 2: Marshaling and Unmarshalling
An alternative approach is to marshal the interface, unmarshal it into a map, add the additional field, and then marshal again:

func printInterface(val interface{}) error {
    data, err := json.Marshal(val)
    if err != nil {
        return err
    }

    v2 := map[string]interface{}{}
    if err := json.Unmarshal(data, &v2); err != nil {
        return err
    }

    v2["Extra"] = "text"
    return json.NewEncoder(os.Stdout).Encode(v2)
}

This method produces the same output as the dynamic type approach.

Conclusion
Both solutions presented here enable the extension of JSON output with arbitrary fields for unknown structs or interfaces. The reflection-based approach provides a type-safe solution, while the marshaling and unmarshaling technique is simpler but potentially slower. The choice of method depends on the specific requirements of the application.

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