json: invalid use of string struct tag

Question

I'm trying to write simple POST serverless Go AWS lambda function.

package main

import (
    "fmt"
)

import (
    "encoding/json"
    "github.com/aws/aws-lambda-go/events"
    "github.com/aws/aws-lambda-go/lambda"
)

// RequestBodyType is our self-made struct to process JSON request from Client
type RequestBodyType struct {
    Event       string `string:"event,string,omitempty"`
    EventParams EventParamsType
}

// ResponseBodyType is our self-made struct to build response for Client
type ResponseBodyType struct {
    Event       string `string:"event,string,omitempty"`
    EventParams EventParamsType
}

// Probably problematic struct?
type EventParamsType struct {
    Name string `json:"name,string,omitempty"`
    Age  int64  `json:"age,omitempty"`
}

// Handler function Using AWS Lambda Proxy Request
func Handler(request events.APIGatewayProxyRequest) (events.APIGatewayProxyResponse, error) {

    // RequestBody will be used to take the json response from client and build it
    requestBody := RequestBodyType{
        Event: "",
        EventParams: EventParamsType{
            Name: "",
            Age:  0,
        },
    }

    // Unmarshal the json, return 404 if error
    err := json.Unmarshal([]byte(request.Body), &requestBody)
    if err != nil {
        return events.APIGatewayProxyResponse{Body: err.Error(), StatusCode: 404}, nil
    }

    // We will build the BodyResponse and send it back in json form
    responseBody := &ResponseBodyType{
        Event:       requestBody.Event,
        EventParams: EventParamsType{
            Name: requestBody.EventParams.Name,
            Age: requestBody.EventParams.Age,   
        },
    }

    fmt.Println("RESPONSE BODY")
    fmt.Println(responseBody)

    // Marshal the response into json bytes, if error return 404
    response, err := json.Marshal(&responseBody)
    if err != nil {
        return events.APIGatewayProxyResponse{Body: err.Error(), StatusCode: 404}, nil
    }

    //Returning response with AWS Lambda Proxy Response
    return events.APIGatewayProxyResponse{Body: string(response), StatusCode: 200}, nil
}

func main() {
    lambda.Start(Handler)
}

Everything works fine if I make curl request with single JSON object key, for example:

curl -X POST https://my.url/dev/event  -d '{"event": "test"}'

And I get response

{"Event":"test","EventParams":{}

But if I make request with nested json object, for example:

curl -X POST https://my.url/dev/event  -d '{"event": "test","eventParams": {"name": "peter","age": 13}}'

Then I get response

json: invalid use of ,string struct tag, trying to unmarshal "peter" into string

I believe I have probably design EventParamsType wrong way? Or am I building ResponseBodyType wrong way?

Answer 1

As the error says, your use of ,string is invalid for your JSON input. Remove it:

    Name string `json:"name,omitempty"`

,string can be valid in a JSON tag, and it indicates that the number should be marshaled as a string literal. In the case of a value which is already a string, that means it expects a JSON-quoted string (which your input apparently is not).

This is explained in the docs:

The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types. This extra level of encoding is sometimes used when communicating with JavaScript programs:

Int64String int64 `json:",string"`

See in the playground for more details.

---

Also, as @Adrian has pointed out, string: is a meaningless tag (for the purpose of JSON (un)marshaling, anyway). You probably want json: instead of string: (although some library may use a tag called string:... 🤷