How to unmarshal json into a struct without mapping all elements while retaining all information

Question content

I receive a JSON object from a queue. I want to process part of the object and update it before sending to another queue.

If I have an input

{
  "one": "data1",
  "two": "data2",
  "three": "data3",
  ...
}

I want to take one and two and calculate some information before adding a new output and sending a similar message

{
  "one": "data1UPDATED",
  "two": "data2",
  "three": "data3",
  ...
  "extra_info": "extra"
}

To be able to process the data better, I want to unmarshal the JSON into a similar structure

type Message struct {
 One string `json:one`
 Two string `json:two`
}

But don't want to map all the fields because many of them are irrelevant to this application and others may change.

Things I've Tried

I tried mapping all fields to json.RawMessage

type Message struct {
 One string `json:"one"`
 Two string `json:"two"`
 ExtraFields json.RawMessage `json:"-"`
}

However, when marshalling the structure, fields contained in ExtraFields are not included.

{
  "one": "data1",
  "two": "data2"
}

I also tried writing a custom unmarshalling that stores the complete message in a struct, with the useful message in one element and the other information in another field

type TmpMessage struct{
  Msg Message
  Others json.RawMessage
}

But this is getting very confusing and I'm hoping for a cleaner solution.

Is there a way to do this, or is the only option without unmarshalling into a struct and just using the raw map[string] interface{}?

Correct answer

You can write a custom unmarshaler, for example:

https://www.php.cn/link/a73d9b34d6f7c322fa3e34c633b1297d

func (m *Message) UnmarshalJSON(data []byte) error {
  tmp := make(map[string]interface{})
  if err := json.Unmarshal(data, &tmp); err != nil {
    return err
  }
  t := reflect.TypeOf(*m)
  for i := 0; i < t.NumField(); i++ {
    f := t.Field(i)
    if f.Name != "ExtraFields" {
      jsonTag := f.Tag.Get("json")
      // this part should probably by a type switch...
      reflect.ValueOf(m).Elem().Field(i).SetString(tmp[jsonTag].(string))
      delete(tmp, jsonTag)
    }
  }
  var err error
  m.ExtraFields, err = json.Marshal(tmp)
  return err
}

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