How do I read write a content item in a separate database transaction to that of the request - orchardcms

Question

I have a Purchase Order content type in my Orchard application. Among other properties it has a PurchaseOrderNumber. The purchase order number is assigned when the user saves the purchase order for the first time. I use a custom controller and views for implementing the purchase order CRUD operations.
I have a purchase order number definition part which is attached to a company content type where the next purchase order number, a prefix and padding is saved. So when the system generates the next purchase order number, the prefix (e.g. PO) is used together with the next number (e.g. 123) and the padding (e.g. 5) to generate a string - e.g. PO00123.
When the purchase order number is generated the next purchase order number stored in the purchase order definition part attached to the company content item is incremented and saved so that when a user creates another purchase order it will be assigned the next number.
My challenge here is to prevent duplicate purchase order numbers from being assigned if two users create a new purchase order at the same time.
I was thinking of creating an ISingletonDependency that uses lock (_lock) {...} to wrap code that will generate the next number. This way multiple request can ask for the next number and always get the next unique number. How do I implement this though? I can't figure out how to get access to an IContentManager that has its own database transaction.
Or is there a different pattern that I should rather use?

Answer 1

I figured it out after looking at the Orchard.Tasks.Locking.Services.DistributedLockService class. You need to take a dependency on ILifetimeScope and then resolve ITransactionManager and IContentManager.
lock (_lock) {
using (var childLifetimeScope = _lifetimeScope.BeginLifetimeScope()) {
var transactionManager = childLifetimeScope.Resolve<ITransactionManager>();
var contentManager = childLifetimeScope.Resolve<IContentManager>();
try {
transactionManager.RequireNew(IsolationLevel.Serializable);
var contentItem = contentManager.GetLatest(contentItemId);
var number = CompileNewNumber(contentItem);
contentManager.Publish(contentItem);
return number;
}
catch (Exception exception) {
Logger.Error(exception, "Error compiling next number.");
transactionManager.Cancel();
return "";
}
}
}