Efficient transaction wrapper function with mongodb go driver - mongodb

Question

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 :
Parse the request content to get the user id
Create a session to perform the transaction
Declare the callback function using the id argument
Call the callback function from a transaction
Get back the *mongo.SingleResult as an interface{} and parsing it back to its original type
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 ?

Answer 1

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
}