Nhibernate - Query One-To-Many Relationship with Linq - nhibernate

Question

i'd like to query a one to many relationship on an entity. To help explain my problem further, imagine my application has the following entities:
public class User {
public virtual int UserID { get; set; }
public virtual Membership CurrentMembership {
get { return Membership.Single(m => m.IsValid); }
}
public virtual IList<Membership> Membership { get; private set; }
public User() {
Membership = new List<Membership>();
}
}
public class Membership {
public virtual int MembershipID { get; set; }
public virtual User User { get; set; }
public virtual DateTime StartDate { get; set; }
public virtual DateTime? EndDate { get; set; }
public virtual int DaysLeft { get; set; }
public virtual bool IsValid { get; set; }
public Membership() {
}
}
With the following Fluent mapping (the mapping is kind of irrelevant, i've only put this here for guidance):
public class UserMap : ClassMap<User> {
public UserMap() {
Table("Users");
Id(x => x.UserID);
HasMany(x => x.Membership)
.KeyColumn("UserID")
.Inverse()
.Cascade.All();
}
}
public class MembershipMap : ClassMap<Membership> {
public MembershipMap() {
Table("Membership");
Id(x => x.MembershipID);
References(x => x.User);
Map(x => x.StartDate);
Map(x => x.DaysLeft)
.Formula("CASE WHEN EndDate IS NOT NULL AND dbo.DayDiff(GETUTCDATE(), EndDate) > 0 THEN dbo.DayDiff(GETUTCDATE(), EndDate) ELSE 0 END");
// DayDiff is a udf which gets the number of days between two dates
Map(x => x.IsValid)
.Formula("CASE WHEN dbo.GetValidMembershipID(UserID) = MembershipID THEN 1 ELSE 0 END");
// GetValidMembershipID is a udf which works out the valid membership id for this user
}
}
As you can see a User can have many Memberships. The CurrentMembership (property against the user) returns the Membership against the User where the IsValid property is true (this will only be true for a single membership against the user).
Now i'd like to be able to grab all users who's current membership has less than 20 days remaining.
My first attempt was to say:
session.Linq<User>().Where(u => u.CurrentMembership.DaysLeft < 20).ToList();
But this threw the error:
"could not resolve property: CurrentMembership of: User"
This was kind of expected since i didn't use a formula mapping for this property (like i did for the DaysLeft and IsValid properties against the membership). I can't see how you can use formula mapping other than to map strings, ints and bools. Next i tried saying:
session.Linq<User>().Where(u => u.Membership.Single(m => m.IsValid).DaysLeft < 20).ToList();
But this threw the error:
"Object reference not set to an instance of an object."
I know i could query this Membership directly but i've used this as an example of something i do in alot of places. Could anyone suggest an alternative way of mapping the CurrentMembership property which allows me to query it with Linq. Please note that i need optimum performance so converting to a list and working in memory will not suffice.
I'd appreciate the help. Thanks

Answer 1

You are using the old LINQ provider in NHibernateContrib for NHibernate 2.1.
This provider is no longer supported nor has any active effort in it.
The LINQ provider built into NHibernate 3.0 syntax is session.Query<TEntity>() instead of session.Linq<TEntity>().
Now NHibernate 3.0 is final release, so, if you had issues with it in the past, they're likely to have been solved already. I used both for heavy queries and the new one satisfies a lot of scenarios.
Maybe the best is to get the latest trunk source and use it.
I highly recommend you replace your stuff with NHibenrate 3.0. It's "almost" 100% backwards compatible with NHibernate 2.1, and all you should need to change is all session.Linq<TEntity>() to session.Query<TEntity>().
If it still doesn't work, you can try u.Membership.Any(...) or u.Membership.First(...) instead of u.Membership.Single(...) in the where condition.
If this still doesn't work, try to reverse the query. Query on Membership and select membership.User.
CurrentMembership will not work as it's not mapped.