RavenDB查询子集合多态性

public class Section
{
    public string BackgroundColor { get; set; }
    public string Description { get; set; }
    public string DesktopBackgroundImageUrl { get; set; }
    public DateTime? EndDate { get; set; }
    public string MobileBackgroundImageUrl { get; set; }
    public int SortOrder { get; set; }
    public DateTime? StartDate { get; set; }
    public string TextColor { get; set; }
    public string Title { get; set; }
    public SectionType Type { get; set; }
}

public class OfferSection : Section
{
    public IEnumerable<Merchant> Merchants { get; set; }
}

我遇到的问题是,我需要查询这个子集合,获取包含派生类型的文档,然后查询其值。

Merchants 属性不存在

public class Hubs_ByMerchantId : AbstractIndexCreationTask<Hub>
{
    public Hubs_ByMerchantId()
    {
        Map = hubs => from hub in hubs
            select new
            {
                Sections_Merchants_Id = hub.Sections.Where(x => x.Type == SectionType.Offer).SelectMany(x => x.Merchants.Select(y => y.Id))
            };
    }
}

有人能给我指出正确的方向吗?

谢谢