Here's your problem:
var reservations = from u in context.Users.Include("Reservations")<br /> where u.UserId == userId<br /> select u.Reservations;
The
Reservations
members in your
User
object is actually a collection of entities; what your
select
clause actually creates is an
IQueryable<EntitySet<Reservation>>
collection, not an
IQueryable<Reservation>
like you would expect.
If you're trying to aggregate/concatenate the
Reservations
collections from multiple
User
objects, you need to look into using LINQ's
Enumerable.SelectMany
[
^] extension method.