JSON Unknown

Imagine we have following Go structs:

type Config struct {
  Name   string  `json:"name,omitempty"`
  Params []Param `json:"params,omitempty"`
}

type Param struct {
  Name  string `json:"name,omitempty"`
  Value string `json:"value,omitempty"`
}

and following json:

{
  "name": "parabolic",
  "subdir": "pb",
  "params": [{
    "name": "input",
    "value": "in.csv"
  }, {
    "name": "output",
    "value": "out.csv",
    "tune": "fine"
  }]
}

and we do unmarshalling:

cfg := Config{}
if err := json.Unmarshal([]byte(cfgString), &cfg); err != nil {
  log.Fatalf("Error unmarshalling json: %v", err)
}
fmt.Println(cfg)

https://play.golang.org/p/HZgo0jxbQrp

Output would be {parabolic [{input in.csv} {output out.csv}]} which makes sense - unknown fields were ignored.

Question: how to find out which fields were ignored?

I.e. getIgnoredFields(cfg, cfgString) would return ["subdir", "params[1].tune"]

(There is a DisallowUnknownFields option but it's different: this option would result Unmarshal in error while question is how to still parse json without errors and find out which fields were ignored)

(Also see on StackOverflow)

This library attempts to provide an answer to this question.

Usage

Library exports ValidateUnknownFields function which returns list of unknown fields and error:

unknowns, err := ValidateUnknownFields(jsonCfgBytes, cfg)

JSON is valid and has no unknown fields if both returned values are nil

If unknown fields were found full path to the field would be specified, e.g. triggers[1].repoz