Question

I have two classes and mapping for the collection:

class User
{
    Guid ID;
    string Name;
}

class Group
{
    Guid ID;
    string Name;
    IList<User> Members;
}

// GroupMap
HasMany(x=>x.Members).Inverse().Cascade.AllDeleteOrhpan().etc.

This one works. When I add a User to the Members collection NHibernate cascades the operation. Same for deletes and updates.

Now I want to change my model a bit and this will change also the mapping. The collection is IList<Guid>. What I really want is the cascade to remain. That means that I have to do some custom persister or IUserType. The mapping should tell the object type like HasMany(x=>x.Members) and the collection will hold the IDs

class User
{
    Guid ID;
    string Name;
}

class Group
{
    Guid ID;
    string Name;
    IList<Guid> Members;
}

// GroupMap
HasMany<User>(x=>x.Members).Inverse().Cascade.AllDeleteOrhpan().etc.

Any ideas where can I start from? I think that there is no out of the box solution but who knows…

PS: NHib user group: https://groups.google.com/forum/#!topic/nhusers/pSUOaGxdxVM

Answer 1

For the 1st comment: The custom persiter or something else should take care about backreference.

so the first mapping doesnt really work already? You could map a virtual backreference on User and set it through an interceptor, i can post something if needed. However i hardly see any advantage.

For the 2nd: Group and User are two aggregate roots. I want to connect the two ARs only by their identity. The two ARs do not need any other information except the other’s ID.

// readonly mapping
HasMany(x => x.Members).Column("Group_Id").Element("Id").Readonly();

I think you will run into a lot of subtle problems when you have a non readonly collection of guids pointing to Users. For example what sql should be generated for:

var user = new User { Id = Guid.New(), Name = "Joe", Age = 12 };
group.Members.Add(user.Id);

session.SaveOrUpdate(group);
session.Flush();

a) INSERT INTO Users (Id, Group_id) Values(...);
b) nothing
c) INSERT INTO Users (Id, Group_id, Name, Age) Values(...);

or this

group.Members.Remove(someGuid);

session.SaveOrUpdate(group);
session.Flush();

a) DELETE FROM Users WHERE group_id = 1;
b) DELETE FROM Users WHERE id = <guid>;
c) Update Users Set group_id = null WHERE Id = <guid>;