func NewDB(host string, port string, dbName string) (db *DB) {
   db = new(DB)
   db.DbName = dbName
   db.Host = host
   db.Port = port
   db.mongoUrl = "mongodb://" + host + ":" + port
   return db
}

type DB struct {
   DbName   string
   Host     string
   Port     string
   mongoUrl string
   client   *mongo.Client
}

func (d *DB) Connect() (err error) {
   log.Println("connecting to: " + d.mongoUrl)
   ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
   defer cancel()
   d.client, err = mongo.Connect(ctx, options.Client().ApplyURI(d.mongoUrl))
   return err
}

func (d *DB) Disconnect() {
   ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
   defer cancel()
   _ = d.client.Disconnect(ctx)
}

func (d *DB) FindOne(collection string, query interface{}, doc interface{}) (found bool, err error) {
   coll := d.client.Database(d.DbName).Collection(collection)
   ctx := context.Background()
   err = coll.FindOne(ctx, query).Decode(doc)
   if err == mongo.ErrNoDocuments {
      return false, nil
   }
   return err == nil, err
}

func (d *DB) Insert(collection string, doc interface{}) (err error) {
   coll := d.client.Database(d.DbName).Collection(collection)
   ctx := context.Background()
   _, err = coll.InsertOne(ctx, doc)
   return err
} 

func main() {
   db := mongo.NewDB("localhost", "27017", "testdb")

   err := db.Connect()
   if err != nil {
      panic(err)
   }
   defer db.Disconnect()

   item := &struct {
      A map[primitive.ObjectID]interface{} `json:"foo" bson:"foo"`
   }{}

   jsonData := "{ \"foo\" : {\"63ea2082b1ce23422a8bf9ca\" : { \"bar\" : \"\" } } }"

   err = json.Unmarshal([]byte(jsonData), item)
   if err != nil {
      fmt.Println("json unmarshal err", err.Error())
   }

   err = db.Insert("test", item)
   if err != nil {
      fmt.Println("db insert err", err.Error())
   }

   _, err = db.FindOne("test", bson.M{}, item)
   if err != nil {
      fmt.Println("db find err", err.Error())
   }

}

db find error decoding key foo.63ea2082b1ce23422a8bf9ca: the provided hex string is not a valid ObjectID