How to Unmarshal JSON with Arbitrary Key/Value Pairs to a Struct in Go?

Unmarshal JSON with Arbitrary Key/Value Pairs to Struct

Problem

You have a JSON string with a combination of known and unknown fields. You need to parse this JSON and map the known fields to a struct while preserving the unknown fields.

Solution 1: Double Unmarshal

type KnownFields struct {
    Known1 string
    Known2 string
}

type Message struct {
    Known1   string                        `json:"known1"`
    Known2   string                        `json:"known2"`
    Unknowns map[string]interface{} `json:"-"`
}

func ParseMessage(jsonStr string) (*Message, error) {
    var knownFields KnownFields
    if err := json.Unmarshal([]byte(jsonStr), &knownFields); err != nil {
        return nil, err
    }

    var m map[string]interface{}
    if err := json.Unmarshal([]byte(jsonStr), &m); err != nil {
        return nil, err
    }

    delete(m, "known1")
    delete(m, "known2")

    return &Message{
        Known1:   knownFields.Known1,
        Known2:   knownFields.Known2,
        Unknowns: m,
    }, nil
}

Solution 2: Directly Unmarshal to Map[string]interface{}

type Message map[string]interface{}

func ParseMessage(jsonStr string) (*Message, error) {
    var m Message
    if err := json.Unmarshal([]byte(jsonStr), &m); err != nil {
        return nil, err
    }

    return &m, nil
}

Additional Option: Unmarshal to Interface{}

func ParseMessage(jsonStr string) (interface{}, error) {
    var m interface{}
    if err := json.Unmarshal([]byte(jsonStr), &m); err != nil {
        return nil, err
    }

    return m, nil
}

The choice between these options depends on your specific requirements. Solution 1 provides a structured Message type that clearly separates known and unknown fields. Solution 2 offers a more flexible approach, allowing you to access all fields as a map. Solution 3 presents a simplified way to unmarshal the JSON into an untyped interface, which you can then handle as needed.

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