How to Map a ValueObject Collection with Foreign Keys in FluentNHibernate - fluent-nhibernate

Question

I've been looking all over for an example of this, but it seems pretty uncommon. Hopefully some NHibernate guru will know.
I have the following class which, by my understanding of Value Objects, is a Value Object. Assume every user has the ability to assign one or more tags to any Question (think Stack Overflow). The Tags don't need a primary key, but they do hold references to the User and Question, unlike most of the examples of ValueObjects I see out there.
public class Tag : ValueObject
{
public virtual User User { get; set; }
public virtual Question Question { get; set; }
public virtual string TagName { get; set; }
}
public class User
{
public virtual IList<Tag> Tags { get; set; }
}
public class Question
{
public virtual IList<Tag> Tags { get; set; }
}
Anyway, I am getting the following error:
{"The entity 'Tag' doesn't have an Id mapped. Use the Id method to map your identity property. For example: Id(x => x.Id)."}
I have the following Fluent NHibernate mapping for User and Question:
public void Override(AutoMapping<XXX> mapping)
{
mapping.HasMany(x => x.Tags).Component(c =>
{
c.Map(x => x.TagName);
c.Map(x => x.Question);
c.Map(x => x.User);
});
}
As always, any thought greatly appreciated.
Late Update: Okay, so, maybe this isn't a value object. It doesn't need an identity, but I guess it's not something that could be used in multiple places, either. Any way to handle this without forcing a useless Id field on my object?

Answer 1

Try this:
public void Override(AutoMapping<XXX> mapping)
{
mapping.HasMany(x => x.Tags).AsBag().Component(c =>
{
c.Map(x => x.TagName);
c.References(x => x.Question);
c.References(x => x.User);
});
}
but you cant query (list all) tags then because its a value object.