Unmarshaling json in Golang: required field?
There is no tag in the encoding/json
package that sets a field to "required". You will either have to write your own MarshalJSON()
method, or do a post check for missing fields.
To check for missing fields, you will have to use pointers in order to distinguish between missing/null and zero values:
type JsonStruct struct {
String *string
Number *float64
}
Full working example:
package main
import (
"fmt"
"encoding/json"
)
type JsonStruct struct {
String *string
Number *float64
}
var rawJson = []byte(`{
"string":"We do not provide a number"
}`)
func main() {
var s *JsonStruct
err := json.Unmarshal(rawJson, &s)
if err != nil {
panic(err)
}
if s.String == nil {
panic("String is missing or null!")
}
if s.Number == nil {
panic("Number is missing or null!")
}
fmt.Printf("String: %s Number: %f\n", *s.String, *s.Number)
}
Playground
You can also override the unmarshalling for a specific type (so a required field buried in a few json layers) without having to make the field a pointer. UnmarshalJSON is defined by the Unmarshaler interface.
type EnumItem struct {
Named
Value string
}
func (item *EnumItem) UnmarshalJSON(data []byte) (err error) {
required := struct {
Value *string `json:"value"`
}{}
all := struct {
Named
Value string `json:"value"`
}{}
err = json.Unmarshal(data, &required)
if err != nil {
return
} else if required.Value == nil {
err = fmt.Errorf("Required field for EnumItem missing")
} else {
err = json.Unmarshal(data, &all)
item.Named = all.Named
item.Value = all.Value
}
return
}
You can just implement the Unmarshaler interface to customize how your JSON gets unmarshalled.