我正在使用 MongoDB。將數(shù)據(jù)添加到集合的代碼：  type User struct {  Firstname        string             `json:"firstname" bson:"firstname"`  Lastname         *string            `json:"lastname,omitempty" bson:"lastname"`  Username         string             `json:"username" bson:"username"`  RegistrationDate primitive.DateTime `json:"registrationDate" bson:"registrationData"`  LastLogin        primitive.DateTime `json:"lastLogin" bson:"lastLogin"`}var client *mongo.Clientfunc AddUser(response http.ResponseWriter, request *http.Request) {  collection := client.Database("hattip").Collection("user")  var user User  _ = json.NewDecoder(request.Body).Decode(&user)  insertResult, err := collection.InsertOne(context.TODO(), user)  if err != nil {    // here i need to get the kind of error.    fmt.Println("Error on inserting new user", err)    response.WriteHeader(http.StatusPreconditionFailed)  } else {    fmt.Println(insertResult.InsertedID)    response.WriteHeader(http.StatusCreated)  }}func main() {  client = GetClient()  err := client.Ping(context.Background(), readpref.Primary())  if err != nil {    log.Fatal("Couldn't connect to the database", err)  } else {    log.Println("Connected!")  }  router := mux.NewRouter()  router.HandleFunc("/person", AddUser).Methods("POST")  err = http.ListenAndServe("127.0.0.1:8080", router)  if err == nil {    fmt.Println("Server is listening...")  } else {    fmt.Println(err.Error())  }}func GetClient() *mongo.Client {  clientOptions := options.Client().ApplyURI("mongodb://127.0.0.1:27017")  client, err := mongo.NewClient(clientOptions)  if err != nil {    log.Fatal(err)  }  err = client.Connect(context.Background())  if err != nil {    log.Fatal(err)  }  return client}