How Can I Partially Decode and Update JSON in Go While Preserving Unknown Fields?

Partial Decoding and Updating of JSON in Go

When working with JSON objects, it may be necessary to decode only specific values while preserving unknown portions of the object. The encoding/json package in Go, however, tends to truncate fields not present in the struct, leading to data loss when re-encoding.

A Solution

To overcome this issue, a combination of a simple struct and json.RawMessage can be used to preserve unknown fields.

type Color struct {
    Space string
    raw   map[string]json.RawMessage
}

In this struct, the raw field stores the entire JSON object as a RawMessage. When decoding, the UnmarshalJSON method reads the raw field to extract specific values into the struct's defined fields.

func (c *Color) UnmarshalJSON(bytes []byte) error {
    if err := json.Unmarshal(bytes, &c.raw); err != nil {
        return err
    }
    if space, ok := c.raw["Space"]; ok {
        if err := json.Unmarshal(space, &c.Space); err != nil {
            return err
        }
    }
    return nil
}

Similarly, when encoding, the MarshalJSON method serializes the Space field and updates the raw map with the encoded bytes.

func (c *Color) MarshalJSON() ([]byte, error) {
    bytes, err := json.Marshal(c.Space)
    if err != nil {
        return nil, err
    }
    c.raw["Space"] = json.RawMessage(bytes)
    return json.Marshal(c.raw)
}

This approach allows for partial decoding and updating of specific fields, while preserving the unknown portions of the JSON object.

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