如何将enum作为字符串映射为流利的nhibernate？

public enum Color
{
    Red,
    Green,
    Blue
}

public class ColorUserType : IUserType
{
    public new bool Equals(object x, object y)
    {
        if (ReferenceEquals(x, y)) return true;
        if (x == null || y == null) return false;
        return x.Equals(y);
    }

    public int GetHashCode(object x)
    {
        return x.GetHashCode();
    }

    public object NullSafeGet(IDataReader rs, string[] names, object owner)
    {
        var value = NHibernateUtil.String.NullSafeGet(rs, names[0]);
        if (value == null) return null;
        return Enum.Parse(typeof(Color), value.ToString());
    }

    public void NullSafeSet(IDbCommand cmd, object value, int index)
    {
        if (value == null)
        {
            NHibernateUtil.String.NullSafeSet(cmd, null, index);
        }
        else
        {
            NHibernateUtil.String.NullSafeSet(cmd, value.ToString(), index);
        }
    }

    public object DeepCopy(object value)
    {
        return value;
    }

    public object Replace(object original, object target, object owner)
    {
        return original;
    }

    public object Assemble(object cached, object owner)
    {
        return cached;
    }

    public object Disassemble(object value)
    {
        return value;
    }

    public SqlType[] SqlTypes { get; } = { NHibernateUtil.String.SqlType };
    public Type ReturnedType { get; } = typeof(Color);
    public bool IsMutable { get; } = false;
}

public class ProductMap : ClassMap<Product>
{
    public ProductMap()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        Map(x => x.Color, cm =>
        {
            cm.Column("color");
            cm.CustomType<ColorUserType>();
            cm.Not.Nullable();
        });
    }
}