EF 6未映射属性以计算相关实体

Question

我需要一个新属性，在哪里统计相关的注册实体。所以我已经定义了一个未映射的属性来获得免费席位。

public class Offer
{
    public int OfferID { get; set;}

    public int maxRegistration { get; set;}

    public virtual ICollection<Registration> Registrations { get; set; }

    [NotMapped]
    public int FreeSeats
    {
        get
        {
            int registrationCounts = Registrations.Count();
            return maxRegistration - registrationCounts;
        }
    }
}

这给我带来了错误：

There is already an open DataReader associated with this Command which must be closed first  

所以，我重写代码：

MyApp.Context db = new MyApp.Context();  
int registrationCounts = db.Registration.Where(r => r.OfferID == OfferID).Count();  

这是正常的，但速度很慢 是否有更优雅的方法来计算数据集的相关实体？