properly call EF SaveChanges after each request with Autofac managing scope - entity-framework

Question

I would like to put in a bit of infrastructure on my project to SaveChanges on my db context at the end of every request.
So I create a simple piece of Owin middleware
app.Use(async (ctx, req) => {
await req();
var db = DependencyResolver.Current.GetService<MyDbContext>();
await db.SaveChangesAsync();
});
This does not work and throws the error
Instances cannot be resolved and nested lifetimes cannot be created from this LifetimeScope as it has already been disposed.
If I resolve the db before completing the request
app.Use(async (ctx, req) => {
var db = DependencyResolver.Current.GetService<MyDbContext>();
await req();
await db.SaveChangesAsync();
});
It doesn't throw an error but it also doesn't work (as in changes aren't saved to the db and viewing db in the debugger shows the DbSet's Local property throwing an InvalidOperationException about it being disposed.
I've tried with and without async, registering the middleware before and after autofac configuration (app.UseAutofacMiddleware(container)) and resolving the LifetimeScope directly from the Owin environment. All give me the same results.
I've done something like this before with Structuremap, but can't seem to figure the correct way to get Autofac to play nice.

Answer 1

Steven is right about the fact that you should not be committing on the request disposal because you cannot be sure if you really want to commit there, unless you abstract your UoW from DbContext and keep a success attribute there, checking if on disposal and conditionally commit.
For your specific question, there are two things to clarify.
DbContext or UoW need to be registered with InstancePerRequest()
Instead of using Owin middleware, you should use OnRelease(context => context.SaveMyChangesIfEverythingIsOk()) native Autofac API
For example, this is how it would look like for RavenDb
builder.Register(x =>
{
var session = x.Resolve<IDocumentStore>().OpenAsyncSession();
session.Advanced.UseOptimisticConcurrency = true;
return session;
})
.As<IAsyncDocumentSession>()
.InstancePerRequest()
.OnRelease(x =>
{
x.SaveChangesAsync();
x.Dispose();
});