Problem when inserting two consecutive lines to the database - sql-server

Question

I have this function and it is working perfectly
public DemandeConge Creat(DemandeConge DemandeConge)
{
try
{
var _db = Context;
int numero = 0;
//??CompanyStatique
var session = _httpContextAccessor.HttpContext.User.Claims.ToList();
int currentCompanyId = int.Parse(session[2].Value);
numero = _db.DemandeConge.AsEnumerable()
.Where(t => t.companyID == currentCompanyId)
.Select(p => Convert.ToInt32(p.NumeroDemande))
.DefaultIfEmpty(0)
.Max();
numero++;
DemandeConge.NumeroDemande = numero.ToString();
//_db.Entry(DemandeConge).State = EntityState.Added;
_db.DemandeConge.Add(DemandeConge);
_db.SaveChanges();
return DemandeConge;
}
catch (Exception e)
{
return null;
}
}
But just when i try to insert another leave demand directly after inserting one (without waiting or refreshing the page )
An error appears saying that this new demand.id exists
I think that i need to add refresh after saving changes?
Any help and thanks

Answer 1

Code like this:
numero = _db.DemandeConge.AsEnumerable()
.Where(t => t.companyID == currentCompanyId)
.Select(p => Convert.ToInt32(p.NumeroDemande))
.DefaultIfEmpty(0)
.Max();
numero++;
Is a very poor pattern. You should leave the generation of your "numero" (ID) up to the database via an Identity column. Set this up in your DB (if DB First) and set up your mapping for this column as DatabaseGenerated.Identity.
However, your code raises lots of questions.. Why is it a String instead of an Int? This will be a bugbear for using an identity column.
The reason you will want to avoid code like this is because each request will want to query the database to get the "max" ID, as soon as you get two requests running relatively simultaneously you will get 2 requests that say the max ID is "100" before either can reserve and insert 101, so both try to insert 101. By using Identity columns the database will get 2x inserts and give them an ID first-come-first-serve. EF can manage associating FKs around these new IDs automatically for you when you set up navigation properties for the relations. (Rather than trying to set FKs manually which is the typical culprit for developers trying to fetch a new ID app-side)
If you're stuck using an existing schema where the PK is a combination of company ID and this Numero column as a string then about all you can do is implement a retry strategy to account for duplicates:
const int MAXRETRIES = 5;
var session = _httpContextAccessor.HttpContext.User.Claims.ToList();
int currentCompanyId = int.Parse(session[2].Value);
int insertAttemptCount = 0;
while(insertAttempt < MAXRETRIES)
{
try
{
numero = Context.DemandeConge
.Where(t => t.companyID == currentCompanyId)
.Select(p => Convert.ToInt32(p.NumeroDemande))
.DefaultIfEmpty(0)
.Max() + 1;
DemandeConge.NumeroDemande = numero.ToString();
Context.DemandeConge.Add(DemandeConge);
Context.SaveChanges();
break;
}
catch (UpdateException)
{
insertAttemptCount++;
if (insertAttemptCount >= MAXRETRIES)
throw; // Could not insert, throw and handle exception rather than return #null.
}
}
return DemandeConge;
Even this won't be fool proof and can result in failures under load, plus it is a lot of code to work around a poor DB design so my first recommendation would be to fix the schema because coding like this is prone to errors and brittle.