json解析后如何列出未知字段

Question

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)

Answer 1

Not sure if that is the best way but what I did is:

1. If current-level type is map:

   1. Check that all map keys are known.
      • It could be either if keys are struct field names or map keys.
      • If not known - add to list of unknown fields
   2. Repeat recursively for value that corresponds to each key

2. If current level type is array:

   1. Run recursively for each element

Code:

// ValidateUnknownFields checks that provided json
// matches provided struct. If that is not the case
// list of unknown fields is returned.
func ValidateUnknownFields(jsn []byte, strct interface{}) ([]string, error) {
    var obj interface{}
    err := json.Unmarshal(jsn, &obj)
    if err != nil {
        return nil, fmt.Errorf("error while unmarshaling json: %v", err)
    }
    return checkUnknownFields("", obj, reflect.ValueOf(strct)), nil
}

func checkUnknownFields(keyPref string, jsn interface{}, strct reflect.Value) []string {
    var uf []string
    switch concreteVal := jsn.(type) {
    case map[string]interface{}:
        // Iterate over map and check every value
        for field, val := range concreteVal {
            fullKey := fmt.Sprintf("%s.%s", keyPref, field)
            subStrct := getSubStruct(field, strct)
            if !subStrct.IsValid() {
                uf = append(uf, fullKey[1:])
            } else {
                subUf := checkUnknownFields(fullKey, val, subStrct)
                uf = append(uf, subUf...)
            }
        }
    case []interface{}:
        for i, val := range concreteVal {
            fullKey := fmt.Sprintf("%s[%v]", keyPref, i)
            subStrct := strct.Index(i)
            uf = append(uf, checkUnknownFields(fullKey, val, subStrct)...)
        }
    }
    return uf
}

Full version: https://github.com/yb172/json-unknown/blob/master/validator.go