type User struct {
    field.DefaultField `bson:",inline"`
    Name string `bson:"name"`
    Age  int    `bson:"age"`
}
u := &User{Name: "Lucas", Age: 7}
_, err := cli.InsertOne(context.Background(), u)
// Fields with tag createAt、updateAt and _id will be generated automatically 

type User struct {
    Name string `bson:"name"`
    Age  int    `bson:"age"`

    MyId         string    `bson:"myId"`
    CreateTimeAt time.Time `bson:"createTimeAt"`
    UpdateTimeAt int64     `bson:"updateTimeAt"`
}

// Implement CustomFields() field.CustomFieldsBuilder 
// And define the custom fields
func (u *User) CustomFields() field.CustomFieldsBuilder {
    return field.NewCustom().SetCreateAt("CreateTimeAt").SetUpdateAt("UpdateTimeAt").SetId("MyId")
}

u := &User{Name: "Lucas", Age: 7}
_, err := cli.InsertOne(context.Background(), u)
// CreateTimeAt、UpdateTimeAt and MyId will be generated automatically 
  
// suppose Id and ui is ready
err = cli.ReplaceOne(context.Background(), bson.M{"_id": Id}, &ui)
// UpdateTimeAt will update