(Golang) InvalidBSON when using custom MarshalBSON() - mongodb

Question

I am currently migrating some of my Python Services to GoLang, where I encountered some problems with the integration with the existing database.
When it comes to saving, the formatting of the date string is different, and I am not planning to change the format of my database entries.
So I decided to implement a time type, with custom (Un)MarshalBSON() methods, to keep pythons time formatting.
However when saving it, I get the following error:
2022/09/10 20:57:29 (InvalidBSON) Unrecognized BSON type 45 in element with field name 'created_at.09-10 20:57:27.798545' in object with _id: ObjectId('631cde192c2aad6a49bc52af')
I don't know how the field created_at.09-10 20:57:27.798545 came up.
Thank you for advance four your advice, here is my source code:
main.go:
package main
import (
"context"
"example/customdate/user"
"fmt"
"log"
)
func main() {
ctx := context.Background()
repo := user.NewRepository(ctx)
u1 := user.NewUser("Max")
u1, err := repo.Add(ctx, u1)
if err != nil {
log.Fatalln(err)
}
fmt.Println(u1)
}
user/repository.go:
package user
import (
"context"
"log"
"go.mongodb.org/mongo-driver/mongo"
"go.mongodb.org/mongo-driver/mongo/options"
)
type Repository interface {
Get(ctx context.Context) ([]User, error)
Add(ctx context.Context, user *User) (*User, error)
}
type repository struct {
client *mongo.Client
}
func NewRepository(ctx context.Context) repository {
c, err := mongo.NewClient(options.Client().ApplyURI("mongodb://127.0.0.1:27017/"))
if err != nil {
log.Fatalln(err)
}
err = c.Connect(ctx)
if err != nil {
log.Fatal(err)
}
return repository{
client: c,
}
}
func (r *repository) Get(ctx context.Context) ([]User, error) {
result, err := r.client.Database("db").Collection("user").Find(ctx, map[string]interface{}{})
if err != nil {
return nil, err
}
var users []User
if err = result.All(ctx, &users); err != nil {
return nil, err
}
return users, nil
}
func (r *repository) Add(ctx context.Context, user *User) (*User, error) {
result, err := r.client.Database("db").Collection("user").InsertOne(ctx, user)
if err != nil {
return nil, err
}
user.ID = result.InsertedID
return user, nil
}
user/dto.go:
package user
import "time"
type CustomTime struct {
time.Time
}
func (t *CustomTime) MarshalBSON() ([]byte, error) {
str := t.Format("2006-01-02 15:04:05.999999")
return []byte(str), nil
}
func (t *CustomTime) UnmarshalBSON(raw []byte) error {
parsed, err := time.Parse("2006-01-02 15:04:05.999999", string(raw))
if err != nil {
return err
}
t = &CustomTime{parsed}
return nil
}
func Now() CustomTime {
return CustomTime{time.Now()}
}
type User struct {
ID interface{}
Name string `bson:"name"`
CreatedAt CustomTime `bson:"created_at"`
}
func NewUser(name string) *User {
return &User{
Name: name,
CreatedAt: Now(),
}
}