How to Partially Decode and Update JSON Data in Go Without Losing Unknown Fields?

Partially Decoding and Updating JSON in Go

When working with JSON data, it's sometimes necessary to modify only specific values without losing the existing structure. This can be challenging if you don't know the entire structure of the JSON object because Go's encoding/json package truncates fields not included in the target struct.

This article explores a technique that leverages a combination of regular structs and json.RawMessage to achieve partial decoding and updating without losing unknown information.

Partial Decoding

The key to partial decoding is to hold the entire JSON data in a raw field (e.g., map[string]json.RawMessage) and selectively unmarshal specific fields into your struct. This is how it's done:

import "encoding/json"

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

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
}

Here, UnmarshalJSON retrieves the entire JSON data into c.raw, then unmarshals the Space field into c.Space.

Modifying Known Fields

Once you have partially decoded the JSON data, you can modify the specific fields as needed.

Marshaling and Preserving

After modifying your known fields, you need to marshal the updated Color struct back to JSON. To preserve the existing structure and avoid truncation, you write your custom MarshalJSON method:

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)
}

In MarshalJSON, you marshal the modified Space field, then embed it back into the raw map before performing the final marshalling.

Conclusion

This technique allows you to modify specific values in JSON data without losing unknown information, making it suitable for scenarios where the full JSON structure is not available or is subject to change.

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