How to Make a Nullable Field in Proto3 for a HTTP Response?

Question

I want to return an object as an HTTP response where one of its fields is nullable. The problem is proto3 won't let me do it easily. This happens because I parsed a pointer of string to a string, so when the pointer points to null it produces this error runtime error: invalid memory address or nil pointer dereference

I have attempted to solve this by at least these two work-arounds I learned from the Internet.

1. Using oneof

exercise.proto (the message definition)

message ExercisesData {
    string Serial = 1 [json_name="serial"];
    string Title = 2 [json_name="title"];
    oneof OptionalSubmissionSerial {
       string SubmissionSerial = 3 [json_name="submission_serial"];
}

mapper.go (to parse a Go struct to fit the proto message)

exercise := &Exercise.ExercisesData {
           Serial:                   e.Serial,
           Title:                    e.Title,
           OptionalSubmissionSerial: &Exercise.ExercisesData_SubmissionSerial{
                SubmissionSerial: *e.SubmissionInfo.LatestSubmissionSerial,
           },
}

2. Using google/protobuf/wrappers.proto

exercise.proto (the message definition)

import "google/protobuf/wrappers.proto";

message ExercisesData {
    string Serial = 1 [json_name="serial"];
    string Title = 2 [json_name="title"];
    google.protobuf.StringValue SubmissionSerial = 3 [json_name="submission_serial"];
}

mapper.go (to parse a Go struct to fit the proto message)

exercise := &Exercise.ExercisesData {
           Serial:                   e.Serial,
           Title:                    e.Title,
           SubmissionSerial:         &wrappers.StringValue{
                Value: *e.SubmissionInfo.LatestSubmissionSerial,
            },
}

Expected Result

Both ways still produce the same error message, the only difference is the line of code it refers to. That's why I am so helpless. The expected HTTP response would look like this

{
    "status": "success",
    "data": [
        {
            "serial": "EXC-NT2OBHQT",
            "title": "Title of Topic Exercise",
            "submission_serial": null
        }
     ]
}

I really hope anyone can help me to find a way to define a nullable field in proto3 for a Http response and how to parse it from a struct. Thank you!

Answer 1

turns out I find another workaround that actually works! It's using google/protobuf/wrappers.proto but I gotta tweak it a lil' bit in the mapper. Here's how it goes:

exercise.proto (the message definition)

import "google/protobuf/wrappers.proto";

message ExercisesData {
    string Serial = 1 [json_name="serial"];
    string Title = 2 [json_name="title"];
    google.protobuf.StringValue SubmissionSerial = 3 [json_name="submission_serial"];
}

mapper.go (to parse a Go struct to fit the proto message)

import "github.com/golang/protobuf/ptypes/wrappers"

        exercise := &pbExercise.GetExercisesData{
            Serial:              e.Serial,
            Title:               e.Title,
        }

        if e.SubmissionInfo.LatestSubmissionSerial != nil {
            exercise.SubmissionSerial = &wrappers.StringValue{
                Value: *e.LatestSubmissionSerial,
            }
        }