Composite Key/Id Mapping with NHibernate - nhibernate

Question

I have the following tables in my database:
Announcements:
- AnnouncementID (PK)
- Title
AnouncementsRead (composite PK on AnnouncementID and UserID):
- AnnouncementID (PK)
- UserID (PK)
- DateRead
Users:
- UserID (PK)
- UserName
Usually I'd map the "AnnouncementsRead" using a many-to-many relationship but this table also has an additional "DateRead" field.
So far I have defined the following entities:
public class Announcement
{
public virtual int AnnouncementID { get; set; }
public virtual string Title { get; set; }
public virtual IList<AnnouncementRead> AnnouncementsRead { get; private set; }
public Announcement()
{
AnnouncementsRead = new List<AnnouncementRead>();
}
}
public class AnnouncementRead
{
public virtual Announcement Announcement { get; set; }
public virtual User User { get; set; }
public virtual DateTime DateRead { get; set; }
}
public class User
{
public virtual int UserID { get; set; }
public virtual string UserName { get; set; }
public virtual IList<AnnouncementRead> AnnouncementsRead { get; private set; }
public User()
{
AnnouncementsRead = new List<AnnouncementRead>();
}
}
With the following mappings:
public class AnnouncementMap : ClassMap<Announcement>
{
public AnnouncementMap()
{
Table("Announcements");
Id(x => x.AnnouncementID);
Map(x => x.Title);
HasMany(x => x.AnnouncementsRead)
.Cascade.All();
}
}
public class AnnouncementReadMap : ClassMap<AnnouncementRead>
{
public AnnouncementReadMap()
{
Table("AnnouncementsRead");
CompositeId()
.KeyReference(x => x.Announcement, "AnnouncementID")
.KeyReference(x => x.User, "UserID");
Map(x => x.DateRead);
}
}
public class UserMap : ClassMap<User>
{
public UserMap()
{
Table("Users");
Id(x => x.UserID);
Map(x => x.UserName);
HasMany(x => x.AnnouncementsRead)
.Cascade.All();
}
}
However when I run this I receive the following error:
"composite-id class must override Equals(): Entities.AnnouncementRead"
I'd appreciate it if someone could point me in the right direction. Thanks

Answer 1

You should do just what NHibernate is telling you. AnnouncementRead should override Equals and GetHashCode methods. They should be based on fields that are part of primary key

Answer 2

When implementing equals you should use instanceof to allow comparing with subclasses. If Hibernate lazy loads a one to one or many to one relation, you will have a proxy for the class instead of the plain class. A proxy is a subclass. Comparing the class names would fail.
More technically: You should follow the Liskows Substitution Principle and ignore symmetricity.
The next pitfall is using something like name.equals(that.name) instead of name.equals(that.getName()). The first will fail, if that is a proxy.
http://www.laliluna.de/jpa-hibernate-guide/ch06s06.html