NHibernate-删除数据库中不存在的内容

我正在尝试从一对多列表中删除一个项目,并将其保留在数据库中。以下是我定义的实体:

public class SpecialOffer
{
    public virtual int SpecialOfferID { get; set; }
    public virtual string Title { get; set; }
    public virtual IList<SpecialOfferType> Types { get; private set; }

    public SpecialOffer()
    {
        Types = new List<SpecialOfferType>();
    }
}

public class SpecialOfferType
{
    public virtual SpecialOffer SpecialOffer { get; set; }
    public virtual Type Type { get; set; }
    public virtual int MinDaysRemaining { get; set; }

    #region composite id requirements
    public override bool Equals(object obj)
    {
        if (obj == null || !(obj is SpecialOfferType))
            return false;

        var t = (SpecialOfferType)obj;

        return SpecialOffer.SpecialOfferID == t.SpecialOffer.SpecialOfferID && Type.TypeID == t.Type.TypeID;
    }

    public override int GetHashCode()
    {
        return (SpecialOffer.SpecialOfferID + "|" + Type.TypeID).GetHashCode();
    }
    #endregion
}

public class Type
{
    public virtual int TypeID { get; set; }
    public virtual string Title { get; set; }
    public virtual decimal Price { get; set; }
}

具有以下映射:

public class SpecialOfferMap : ClassMap<SpecialOffer>
{
    public SpecialOfferMap()
    {
        Table("SpecialOffers");
        Id(x => x.SpecialOfferID);
        Map(x => x.Title);
        HasMany(x => x.Types)
            .KeyColumn("SpecialOfferID")
            .Inverse()
            .Cascade.All();
    }
}

public class SpecialOfferTypeMap : ClassMap<SpecialOfferType>
{
    public SpecialOfferTypeMap()
    {
        Table("SpecialOfferTypes");
        CompositeId()
            .KeyReference(x => x.SpecialOffer, "SpecialOfferID")
            .KeyReference(x => x.Type, "TypeID");
        Map(x => x.MinDaysRemaining);
    }
}

public class TypeMap : ClassMap<Type>
{
    public TypeMap()
    {
        Table("Types");
        Id(x => x.TypeID);
        Map(x => x.Title);
        Map(x => x.Price);
    }
}

我的问题是,如果我从特殊服务类型集合成功地将其从列表中删除,但当我尝试保存会话时,更改不会持久化到数据库中。我假设这与连接表上的复合id有关,因为我过去已经能够用标准id成功地做到这一点。

如果有人能告诉我我做错了什么,我会很感激的。谢谢