Sorry, I have updated this one multiple times as I am myself finding a solution.

Seems there is many ways to tackle this one, but the most efficient I have found so far is:

Add this under default route:

routes.MapHttpRoute(
    name: "OneLevelNested",
    routeTemplate: "api/{controller}/{customerId}/{action}/{id}",
    defaults: new { id = RouteParameter.Optional }
);

This route will then match any controller action and the matching segment name in the URL. For example:

/api/customers/1/orders will match:

public IEnumerable<Order> Orders(int customerId)

/api/customers/1/orders/123 will match:

public Order Orders(int customerId, int id)

/api/customers/1/products will match:

public IEnumerable<Product> Products(int customerId)

/api/customers/1/products/123 will match:

public Product Products(int customerId, int id)

The method name must match the {action} segment specified in the route.

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Important Note:

From comments

Since the RC you’ll need to tell each action which kind of verbs that are acceptable, ie [HttpGet], etc.