How to Map a ValueObject Collection with Foreign Keys in FluentNHibernate

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 m开发者_运维技巧ore 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?

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.