How Can I Partially Decode and Update JSON Data in Go Without Losing Unknown Information?

Partial Decoding and Updating of JSON in Go

When working with JSON data, it is often necessary to access and modify specific values without having the full structure of the object. The encoding/json package in Go, while providing robust decoding and encoding capabilities, truncates or ignores fields not explicitly defined in the destination struct. This can lead to loss of unknown information upon re-encoding.

Solution: Employing json.RawMessage

To overcome this challenge, it is possible to leverage a combination of a regular struct and json.RawMessage to achieve partial decoding and updating. json.RawMessage represents raw JSON data, allowing any JSON structure to be held without decoding its specific form.

In the following code snippet, the Color struct is defined with a Space field and a raw field of type map[string]json.RawMessage. This enables the storage of the entire JSON object while explicitly unmarshalling only the known field (Space).

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

Unmarshaling and Updating

During the unmarshaling process, the UnmarshalJSON method extracts the Space field from the raw data, if present. The complete raw data is stored in the raw map.

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
}

When updating values, only the known fields need to be modified. In this case, color.Space can be assigned a new value.

Marshaling

During marshaling, the MarshalJSON method retrieves the updated space value and stores it as json.RawMessage in the raw map before encoding the entire object as JSON.

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)
}

Example Usage

The following example demonstrates the partial decoding and updating of a JSON object:

before := []byte(`{"Space": "YCbCr", "Point": {"Y": 255, "Cb": 0, "Cr": -10}}`)

// Decode
color := new(Color)
err := json.Unmarshal(before, color)

// Modify the Space field
color.Space = "RGB"

// Encode
after, err := json.Marshal(color)

The output would be:

{"Point":{"Y":255,"Cb":0,"Cr":-10},"Space":"RGB"}

This approach preserves the unknown structure and information, allowing for partial decoding and updating of JSON objects.

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