Receiving unknown response body

I am implementing the Authorize.net credit card API. The API always gives me a 200 response code regardless if the transaction is successful or it is declined. But it gives the one response body for successful transaction and a different one for declined transaction.

type AuthorizeApprovedResponse struct {
	TransactionResponse struct {
		ResponseCode   string `json:"responseCode"`
		AuthCode       string `json:"authCode"`
		AvsResultCode  string `json:"avsResultCode"`
		CvvResultCode  string `json:"cvvResultCode"`
		CavvResultCode string `json:"cavvResultCode"`
		TransID        string `json:"transId"`
		RefTransID     string `json:"refTransID"`
		TransHash      string `json:"transHash"`
		TestRequest    string `json:"testRequest"`
		AccountNumber  string `json:"accountNumber"`
		AccountType    string `json:"accountType"`
		Messages       []struct {
			Code        string `json:"code"`
			Description string `json:"description"`
		} `json:"messages"`
		UserFields []struct {
			Name  string `json:"name"`
			Value string `json:"value"`
		} `json:"userFields"`
		TransHashSha2                          string `json:"transHashSha2"`
		SupplementalDataQualificationIndicator int    `json:"SupplementalDataQualificationIndicator"`
		NetworkTransID                         string `json:"networkTransId"`
	} `json:"transactionResponse"`
	RefID    string `json:"refId"`
	Messages struct {
		ResultCode string `json:"resultCode"`
		Message    []struct {
			Code string `json:"code"`
			Text string `json:"text"`
		} `json:"message"`
	} `json:"messages"`
}

type AuthorizeDeclinedResponse struct {
	TransactionResponse struct {
		ResponseCode   string `json:"responseCode"`
		AuthCode       string `json:"authCode"`
		AvsResultCode  string `json:"avsResultCode"`
		CvvResultCode  string `json:"cvvResultCode"`
		CavvResultCode string `json:"cavvResultCode"`
		TransID        string `json:"transId"`
		RefTransID     string `json:"refTransID"`
		TransHash      string `json:"transHash"`
		TestRequest    string `json:"testRequest"`
		AccountNumber  string `json:"accountNumber"`
		AccountType    string `json:"accountType"`
		Errors         []struct {
			ErrorCode string `json:"errorCode"`
			ErrorText string `json:"errorText"`
		} `json:"errors"`
		UserFields []struct {
			Name  string `json:"name"`
			Value string `json:"value"`
		} `json:"userFields"`
		TransHashSha2                          string `json:"transHashSha2"`
		SupplementalDataQualificationIndicator int    `json:"SupplementalDataQualificationIndicator"`
		NetworkTransID                         string `json:"networkTransId"`
	} `json:"transactionResponse"`
	RefID    string `json:"refId"`
	Messages struct {
		ResultCode string `json:"resultCode"`
		Message    []struct {
			Code string `json:"code"`
			Text string `json:"text"`
		} `json:"message"`
	} `json:"messages"`
}

Here is my problem, which struct to use. I was thinking of trying an interface{} and then try to cast it to a struct?

err := json.Unmarshal(b, &whichStructToUse)
	if err != nil {
		panic(err.Error())
	}

Any advice on how to Unmarshal the response when I don't know which struct to use?

> The API always gives me a 200 response code regardless if the transaction is successful or it is declined.

I feel your pain.

There's only one difference between the two responses, success has Messages and failure has Errors. Combine them.

type CommonResponse struct {
    TransactionResponse struct {
        ResponseCode   string `json:"responseCode"`
        AuthCode       string `json:"authCode"`
        AvsResultCode  string `json:"avsResultCode"`
        CvvResultCode  string `json:"cvvResultCode"`
        CavvResultCode string `json:"cavvResultCode"`
        TransID        string `json:"transId"`
        RefTransID     string `json:"refTransID"`
        TransHash      string `json:"transHash"`
        TestRequest    string `json:"testRequest"`
        AccountNumber  string `json:"accountNumber"`
        AccountType    string `json:"accountType"`
        Messages       []struct {
            Code        string `json:"code"`
            Description string `json:"description"`
        } `json:"messages"`
        Errors         []struct {
            ErrorCode string `json:"errorCode"`
            ErrorText string `json:"errorText"`
        } `json:"errors"`
        UserFields []struct {
            Name  string `json:"name"`
            Value string `json:"value"`
        } `json:"userFields"`
        TransHashSha2                          string `json:"transHashSha2"`
        SupplementalDataQualificationIndicator int    `json:"SupplementalDataQualificationIndicator"`
        NetworkTransID                         string `json:"networkTransId"`
    } `json:"transactionResponse"`
    RefID    string `json:"refId"`
    Messages struct {
        ResultCode string `json:"resultCode"`
        Message    []struct {
            Code string `json:"code"`
            Text string `json:"text"`
        } `json:"message"`
    } `json:"messages"`
}

Then use that to unmarshall and check for Errors.

	var response CommonResponse;
    json.Unmarshal([]byte(jsonString), &response)
	if len(response.Error) == 0 {
		fmt.Println("Success!")
	} else {
		fmt.Println("Error!")
	}

For the more general case, you can unmarshall to a map[string]interface{}.

var result map[string]interface{}
json.Unmarshal([]byte(jsonString), &result)

Demonstration.