How to build a nested json/object recursively from a nested struct in Golang

Question

How do i use the following struct to build a nested json recursively?

The nested struct can go to as many levels as it can.

Both the sample struct and json is mentioned below.

I'm having trouble in building a nested json/object dynamically.

I used the reflect package to access the struct.

I'm able to read through the data but not able to build the same.

type Data struct {
    ID string 
    Name string 
    Types *Details
}

type Details struct {
    Customer int32 
    Countries   int32       
}

To:

{
  "Name":"Data",
  "Fields":[{
    "Name":"ID",
    "Type":"string"
  },
  {
    "Name":"Name",
    "Type":"string"
  },
  {
    "Name":"Details",
    "Type":"struct",
    "Fields":[{
      "Name":"Customer",
      "Type":"int32"
    },{
      "Name":"Countries",
      "Type":"int32"
    }]
}

And whatever I have done so far, I have attached below:

package main

import (
    "encoding/json"
    "fmt"
    "reflect"
)

func check(e error) {
    if e != nil {
        panic(e)
    }
}

func getFields(t reflect.Type, prefix string) {
    switch t.Kind() {
    case reflect.Ptr:
        getFields(t.Elem(), "")
    case reflect.Struct:
        buildRecursiveFunction(t)
    }
}

func buildRecursiveFunction(t reflect.Type) map[string]interface{} {
    var jsonArr []interface{}
    temp := make(map[string]interface{})
    for i := 0; i < t.NumField(); i++ {
        sf := t.Field(i)
        if sf.Name != "state" && sf.Name != "unknownFields" && sf.Name != "sizeCache" {
            obj := make(map[string]interface{})
            varName := sf.Name
            varType := sf.Type.Kind()
            obj["Name"] = varName
            obj["Type"] = varType.String()

            if varType.String() == "ptr" {
                obj["Fields"] = buildRecursiveFunction(sf.Type.Elem())
            } else {
            }
            jsonArr = append(jsonArr, obj)
        }
    }
    jsonArrVal, _ := json.Marshal(jsonArr)
    fmt.Println(string(jsonArrVal))
    return temp

}

func main() {
    getFields(reflect.TypeOf(&DeviceEnv{}), "")
    // json.Marshal(AllAttributes)
}

Any help is appreciated.

Note that in your "TO" example you've shown the string and bool fields as "Type": "struct". Presumably they should be "string" and "bool". — Ben Hoyt
– Ben Hoyt, Commented Jun 28, 2022 at 0:08

Ben Hoyt · Accepted Answer · 2022-06-28 00:18:34Z

I've hacked around your code and got it (kind of) working here, but it doesn't seem like it's structured in the best way. Better to return and build a recursive object than trying to print it (with a prefix?) as you go.

What about defining a TypeInfo struct and having a recursive function that populates that? I think that leads to a clear structure, and it allows the caller to JSON-marshal the result as they want to:

func main() {
    info := MakeTypeInfo("DeviceEnvironment", DeviceEnv{})
    b, err := json.MarshalIndent(info, "", "  ")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("%s\n", b)
}

type TypeInfo struct {
    Name   string     `json:"Name"`
    Type   string     `json:"Type"`
    Fields []TypeInfo `json:"Fields,omitempty"`
}

func MakeTypeInfo(name string, value interface{}) TypeInfo {
    return makeTypeInfo(name, reflect.TypeOf(value))
}

func makeTypeInfo(name string, t reflect.Type) TypeInfo {
    kind := t.Kind()
    switch kind {
    case reflect.Struct:
        var fields []TypeInfo
        for i := 0; i < t.NumField(); i++ {
            field := t.Field(i)
            fields = append(fields, makeTypeInfo(field.Name, field.Type))
        }
        return TypeInfo{Name: name, Type: kind.String(), Fields: fields}
    case reflect.Pointer:
        return makeTypeInfo(name, t.Elem())
    default:
        return TypeInfo{Name: name, Type: kind.String()}
    }
}

Note that I haven't done the field filtering (eg: "unknownFields") that you've shown in your code -- shouldn't be hard to add inside the reflect.Struct case though.

Full example in Go Playground.

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