How to handle duplicate unique index error? - mongodb

Question

I'm using MongoDB. Code to add data to the collection:
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.Client
func 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
}
If I add a record with a username that already exists in the database, I get -
Error on inserting new user multiple write errors: [{write errors:
[{E11000 duplicate key error collection: hattip.user index:
username_unique dup key: { username: "dd" }}]}, {}]
in the line fmt.Println("Error on inserting new user", err) The record with the string dd in the username field is already there, and the username field is a unique index.
I want to be sure that the error is exact E11000 error (a repeating collection of key errors).
So far i compare err to whole error string that appears on duplication of a unique field, but it's completely wrong. If there is a way to get error code from err object, or there are other ways to solve this problem?
Also, i found mgo package, but to use it properly i have to learn it, rewrite current code and so on, but honestly, it looks good:
if mgo.IsDup(err) {
err = errors.New("Duplicate name exists")
}

Answer 1

According to the driver docs, InsertOne may return a WriteException, so you can check if the error is a WriteException, and if it is so, then check the WriteErrors in it. Each WriteError contains an error code.
if we, ok:=err.(WriteException); ok {
for _,e:=range we.WriteErrors {
// check e.Code
}
}
You can write an IsDup based on this.