将Project方法与嵌套数组一起使用时，MongoDB驱动程序返回null

Question

考虑此模型结构：

allow

当我尝试检索特定类别的产品时，它返回null：

public class Entity : Document
    {
        [BsonRepresentation(BsonType.Int32)]
        public int CreateUserId { get; set; }
        [BsonRepresentation(BsonType.Int32)]
        public int ChangeUserId { get; set; }
        [BsonDateTimeOptions(Kind = DateTimeKind.Utc, Representation = BsonType.DateTime)]
        public DateTime ChangeDate { get; set; }
    }

public class TrackedEntity : Entity
    {
        public bool Deleted { get; set; }
    }

public class Shop : TrackedEntity
    {
        public string Name { get; set; }
        public string Address { get; set; }
        public string MobilePhone { get; set; }
        public string PhoneNumber { get; set; }
        public string ShopTelegramToken { get; set; }
        public string ShopAndroidId { get; set; }
        public string TelegramURL { get; set; }
        public string TelegramName { get; set; }
        public List<Category> CategoryList { get; set; } = new List<Category>();
    }

public class Category : TrackedEntity
    {
        [BsonRepresentation(BsonType.String)]
        public string Name { get; set; }
        public string Description { get; set; }
        [BsonRepresentation(BsonType.String)]
        public string ParentCategoryId { get; set; }
        [BsonRepresentation(BsonType.String)]
        public string ShopId { get; set; }

        public List<Product> ProductList { get; set; } = new List<Product>();
    }

public class Product : TrackedEntity
    {
        [BsonRepresentation(BsonType.String)]
        public string Name { get; set; }
        [BsonRepresentation(BsonType.String)]
        public string ProductCategoryId { get; set; }
        public string ImageId { get; set; }
        [BsonRepresentation(BsonType.String)]
        public string ShopId { get; set; }
        public List<Property> PropertyList { get; set; } = new List<Property>();
    }

我认为问题出在Project方法中的ElementAt方法上，因为不使用返回值就不会为空。