I have a simple application that must assign a sequential number without gaps or duplicates to each Document that is created i.e. 1, 2, 3 etc. A different document type will have a different numering sequence.

DocumentTypes is a database table which stores the different types of documents e.g. Book, Recipe, WhitePaper

Documents is a database table which stores the document info including its sequential ID

The database relationship is one DocumentType can have many Documents but each Document will have only one DocumentType

If DocumentType.LastNumber holds the last number assigned for a particular document type, my plan was:

// create a transaction to lock the DocumentType row being assigned so no duplicates
var transaction = dbContext.Database.BeginTransaction(IsolationLevel.Serializable);

// get the appropriate document type from the db
var documentType = dbContext.DocumentTypes.Single(t => t.Id == 1);

// increment the last document number
documentType.LastNumber++;    

// create the new document with the new number
documentType.Documents.Add(new Document{
    Id = documentType.LastNumber
});

dbContext.SaveChanges();

// commit the transaction and release the db lock
transaction.Commit();

However, what Im finding is that I get duplicate numbers for a given document type (when I call this method asynchronously via API)

I suspect I have misunderstood how database transactions and IsolationLevel.Serializable work. Any feedback would be greatly appreciated.