Question

I’m building a simple multi-user (multi-tenant?) App with ASP.NET MVC3 and EF4, one database, one code base, all users access the app using the same URL. Once a User is logged in they should only have access to their data, I’m using the default asp.NET membership provider and have added a ‘UserId’ Guid field on each of the data tables. Obviously I don’t want user A to have any access to user B’s data so I have been adding the following to nearly every action on my controllers.

public ActionResult EditStatus(int id)
    {
        if (!Request.IsAuthenticated)
            return RedirectToAction("Index", "Home");

        var status = sService.GetStatusById(id);

        // check if the logged in user has access to this status
        if (status.UserId != GetUserId())
            return RedirectToAction("Index", "Home");
    .
    .
    .
    }

    private Guid GetUserId()
    {
        if (Membership.GetUser() != null)
        {
            MembershipUser member = Membership.GetUser();
            Guid id = new Guid(member.ProviderUserKey.ToString());
            return id;
        }
        return Guid.Empty;
    }

This repetition is definitely feeling wrong and there must be a more elegant way of ensuring my users can’t access each other’s data – what am I missing?

Answer 1

what am I missing?

A custom model binder:

public class StatusModelBinder : DefaultModelBinder
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        // Fetch the id from the RouteData
        var id = controllerContext.RouteData.Values["id"] as string;

        // TODO: Use constructor injection to pass the service here
        var status = sService.GetStatusById(id);

        // Compare whether the id passed in the request belongs to 
        // the currently logged in user
        if (status.UserId != GetUserId())
        {
            throw new HttpException(403, "Forbidden");
        }
        return status;
    }

    private Guid GetUserId()
    {
        if (Membership.GetUser() != null)
        {
            MembershipUser member = Membership.GetUser();
            Guid id = new Guid(member.ProviderUserKey.ToString());
            return id;
        }
        return Guid.Empty;
    }
}

and then you would register this model binder in Application_Start:

// Could use constructor injection to pass the repository to the model binder
ModelBinders.Binders.Add(typeof(Status), new StatusModelBinder());

and finally

// The authorize attribute ensures that a user is authenticated. 
// If you want it to redirect to /Home/Index as in your original
// example if the user is not authenticated you could write a custom
// Authorize attribute and do the job there
[Authorize]
public ActionResult EditStatus(Status status)
{
    // if we got that far it means that the user has access to this resource
    // TODO: do something with the status and return some view
    ...
}

Conclusion: We’ve put this controller on a diet which is the way controllers should be 🙂