How to take input as json string and later convert it to any object based on condition

Here I want to take DefaultResponse as String but later validate if it's a valid json before mapping to any json struct.

Complete code with validation can be found here :
https://go.dev/play/p/knGNMj1QG3l

type AddEndpointRequest struct {
	Method            string `json:"method"`
	ContentType       int    `json:"contentType"`
	DefaultStatusCode int    `json:"defaultStatusCode"`
	DefaultResponse   string `json:"defaultResponse"`
}

I tried different option but none of them are working

1. if I pass this : “defaultResponse”:{“title”:“Buy cheese and bread for breakfast.”}
   Getting error : json: cannot unmarshal object into Go struct field AddEndpointRequest.defaultResponse of type string

2)
body := {"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":"[{"p":"k"}]"}

Error : invalid character 'p' after object key:value pair

3)
body := {"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":"{"p":"k"}"}
./prog.go:21:117: syntax error: unexpected { at end of statement

4. body := {"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":"{/"p/":/"k/"}"}
Error : ./prog.go:21:130: syntax error: unexpected literal "} at end of statement

And many more

Option A:

Declare the field as a string. Use a valid JSON string as the field value in the document. Note the escaping of the quotes in the string.

type AddEndpointRequest struct {
	Method            string          `json:"method"`
	ContentType       int             `json:"contentType"`
	DefaultStatusCode int             `json:"defaultStatusCode"`
	DefaultResponse   string          `json:"defaultResponse"`
}
…
body := `{"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":"{\"p\":\"k\"}"}`

Convert the field to a []byte to unmarshal:

err := json.Unmarshal([]byte(request.DefaultResponse), &data)

https://go.dev/play/p/ailgQQ3eQBH

Option B:

Declare the field as json.RawMessage. Use any valid JSON in the document.

type AddEndpointRequest struct {
	Method            string          `json:"method"`
	ContentType       int             `json:"contentType"`
	DefaultStatusCode int             `json:"defaultStatusCode"`
	DefaultResponse   json.RawMessage `json:"defaultResponse"`
}
…
body := `{"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":{"p":"k"}}`

The call to json.Unmarshal([]byte(body), &request) validates the json.RawMessage field. If the call to json.Unmarshal does not return an error, then the application is assured that AddEndpointRequest.DefaultResponse contains valid JSON.

Unmarshal the field like this:

err := json.Unmarshal(request.DefaultResponse, &data)

https://go.dev/play/p/Xd_gWzJmvC_K

If you want to set a struct in defaultResponse, I think doing it like this should work.

package main

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

type AddEndpointRequest struct {
	Method            string `json:"method"`
	ContentType       int    `json:"contentType"`
	DefaultStatusCode int    `json:"defaultStatusCode"`
	DefaultResponse   string `json:"defaultResponse"`
}

func main() {

	body := `{"method":"GET","contentType":1,"defaultWaitTimeInMillis":100,"defaultStatusCode":200,"defaultResponse":{"title":"Buy cheese and bread for breakfast."}}`
	fmt.Println("Hello GO")

	err := validateBody(body)
	if err != nil {
		fmt.Println("erro102")
		fmt.Println(err)

	}
}

func validateBody(body string) error {
	var data map[string]interface{}
	err := json.Unmarshal([]byte(body), &data)
	if err != nil {
		return errors.New("invalid json body provided for the request")
	}

	fmt.Println("data===>", data)

	fmt.Println("defaultResponse===>", data["defaultResponse"].(map[string]interface{})["title"])

	return nil
}