NHibernate QueryCache in Multiuser-Environment - fluent-nhibernate

Question

For our web-application (ASP.NET) we're using Fluent NHibernate (2.1.2) with 2nd-Level caching not only for entities, but also for queries (generating queries with the criteria API). We're using the Session-Per-Request pattern and one SessionFactory applicationwide, so the cache serves all Nhibernate-Sessions.
Problem:
We have to deal with different "Access-Rigths" per user on the data-objects in our legacy-database (Oracle) - that is, views constrain the returning data per user-rights.
So there's the situation, where for example the same view is queried by our criteria with the excact same query, but returns a different resultset, depending on the user-Rights.
Now, to gain performance, the mentioned query is cached. But this gives us the problem, that when the query is first fired from an action of user A, it caches the resulting ID's, which are the ID's to which user A has access rights. Shortly after, the same query is fired from an action of user B and Nhibernate then picks the cached ID's from the first call (from user A) and tries to get the corresponding entities, to which User B doesn't have access-rights (or maybe not for all of them). We're checking the rights with event-listeners, so our appliction throws an access-right-exception in the mentioned case.
Thoughts:
Not caching the queries could be an option against this. But performance is cleary an issue in our application, so it would be really desirable to have cached queries user-wise.
We even thought about a SessionFactory per user, to have a cache per user, sort of. But this has clearly an impact on ressources, is somewhat of an overkill and honestly isn't an option, because
there are entities, which have to be accessed, and are manipulated, by multiple users (think of a user-group), creating issues with stale data in the "individual caches" and so on. So that's a no-go.
What would be a valid solution for this? Is there something like "best practice" for such a situation?
Idea:
As I was stuck with this yesterday, seeing no way out, I slept over it, and today I came up with some sort of a "hack".
As NHibernate caches the query by query-text and parameters ("clauses"), I thought about a way, to "smuggle" something user-dependent in that signature of the queries, so it would
cache every query per user, but would not alter the query itself (concerning the result of the query).
So "creativity" guided me to this (example-code):
string userName = GetCurrentUser();
ICriteria criteria = session.CreateCriteria(typeof (EntityType))
.SetCacheable(true)
.SetCacheMode(CacheMode.Normal)
.Add(Expression.Eq("PropertyA", 1))
.Add(Expression.IsNotNull("PropertyB"))
.Add(Expression.Sql(string.Format("'{0}' = '{0}'", userName)));
return criteria.List();
This line:
.Add(Expression.Sql(string.Format("{0} = {0}", userName)))
results in a where-clause, which always evaluates to true, but "changes" the query from Nhibernate's viewpoint, so it caches per separate "userName".
I know, it's kind of ugly and I'm not really pleased with it.
Does anybody knows any alternative approach?
thanks in advance.