What's the best way to maintain un-parsed JSON fields in Go?

Question

I'd like to decode a JSON blob into a Go struct, manipulate on it, and encode it back to JSON. However, there are dynamic fields in the JSON that aren't relevant to my struct and I want to maintain them when I serialize back to JSON.

For example:

{ "name": "Joe Smith", 
  "age": 42, 
  "phone": "614-555-1212", 
  "debug": True, 
  "codeword": "wolf" }

type Person struct {
    Name string
    Age uint
    Phone string
}

var p Person
json.Unmarshal(data, &p)
// Happy birthday
p.Age++
data, _ = json.Marshal(p)

// Any way to maintain the "debug" and "codeword" fields -- which might not
// be known ahead of time?

I know one possibility is to decode everything into a map[string]interface{} but boy, do things get ugly when you do that.

Is there any way to have the best of both worlds?

Answer 1

With encoding/json there's no way to do decode a struct and save unknown fields at the same level for posterior re-encoding. What you can do is choose not to decode part of the struct by using the json.RawMessage type, along the lines of:

type Person struct {
    Name    string
    Address json.RawMessage
}

You might workaround that by implementing your own Unmarshaler that decodes the document into a map and saves the unknown keys in a field of the struct, and then have a counterpart Marshaler that puts the fields back before marshaling.

Just out of curiosity, the feature you're looking for does exist in labix.org/v2/mgo/bson, via the inline tag flag, and the goal was to solve precisely the use case you're describing.