Efficient transaction wrapper function with mongodb go driver

I'm currently integrating the transaction logic into my go+mongodb api.

I already created this example endpoint. It allows you to retrieve a user document and send it back to the client with json encoding.

func GetUser(w http.ResponseWriter, r *http.Request) {
	id := mux.Vars(r)["user-id"]
	objectId, _ := primitive.ObjectIDFromHex(id)

	user, err := UserById(objectId)
	if err != nil {
		// ...
	}

	// do some stuff with the user, whatever

    // eventually send it back
	json.NewEncoder(w).Encode(user)
}

func UserById(id primitive.ObjectID) (models.StoredUser, error) {
	session, err := mongoClient.StartSession()
	if err != nil {
		return models.StoredUser{}, fmt.Errorf("failed starting session for transaction | %s", err.Error())
	}
	defer session.EndSession(context.TODO())

	callback := func(ctx mongo.SessionContext) (any, error) {
		res := usersCollection.FindOne(
			ctx,
			bson.M{
				"_id": id,
			},
		)

		if res.Err() != nil {
			return models.StoredUser{}, fmt.Errorf("failed querying db | %s", res.Err())
		}

		return res, nil
	}

	result, err := session.WithTransaction(context.TODO(), callback)
	if err != nil {
		return models.StoredUser{}, fmt.Errorf("failed executing transaction | %s", err.Error())
	}

	asserted := result.(*mongo.SingleResult)
	var ret models.StoredUser
	if err := asserted.Decode(&ret); err != nil {
		return models.StoredUser{}, fmt.Errorf("failed parsing user data in struct | %s", err.Error())
	}

	return ret, nil
}

Here are the big steps :

1. Parse the request content to get the user id
2. Create a session to perform the transaction
3. Declare the callback function using the id argument
4. Call the callback function from a transaction
5. Get back the *mongo.SingleResult as an interface{} and parsing it back to its original type
6. Decode the bson document contained in the *mongo.SingleResult to put it in the return struct

This function works, but is very verbose. The code is very duplicated.

I wonder if there is a way of not repeating the same code for each function I wanna make. My previous wrapper function attempts didn't lead to anything, as I actually need the variables where they are now at each call.

Still, the steps 2 and 5 especially seem very redundant and inefficient.

Anyone got any idea ?

Ok I found the following :

func Transaction(callback func(ctx mongo.SessionContext) (any, error)) (any, error) {
	session, err := mongoClient.StartSession()
	if err != nil {
		return nil, fmt.Errorf("failed creating session | %s", err.Error())
	}

	defer session.EndSession(context.TODO())

	res, err := session.WithTransaction(ctx, callback)
	if err != nil {
		return nil, fmt.Errorf("failed executing transaction | %s", err.Error())
	}

	return res, nil
}

Let's say i want then to fetch user objects :

func GetUsers() ([]models.User, error) {
	callback := func(ctx mongo.SessionContext) (any, error) {
		res, err := usersCollection.Find(ctx, bson.M{})
		if err != nil {
			return nil, fmt.Errorf("failed querying users collection | %s", err.Error())
		}

		var ret []models.User
		if err := res.All(context.TODO(), &ret); err != nil {
			return nil, fmt.Errorf("failed parsing results in struct | %s", err.Error())
		}

		return ret, nil
	}

	result, err := Transaction(callback)
	if err != nil {
		return []models.User{}, fmt.Errorf("failed executing transaction | %s", err.Error())
	}

	classes, _ := result.([]models.StoredClass)
	return classes, nil
}