如何在C＃中使用組中的X個元素lambda

Question

我想從groupby中取出最後4個元素，然後將其添加到列表中。我使用MySQL作爲數據庫和asp MVC5。我嘗試這樣做：

List<Payments> payments= db.Payments 
    .Where(e => e.Account.Active==true) 
    .GroupBy(e => e.IdAccount) 
    .Select(e => e.OrderBy(x => x.PaidDate).Take(4)) 
    .SelectMany(e => e).ToList(); 

我得到了以下錯誤：

Unknown column 'Join2.IdAccount' in 'where clause'

這是支付類：

public partial class Payment 
{ 
    public int Id { get; set; } 

    [ForeignKey("Account")] 
    [DisplayName("ID Account")] 
    public int IdAccount { get; set; } 

    [ForeignKey("Client")] 
    [DisplayName("ID Client")] 
    public int IdClient { get; set; } 

    public Nullable<decimal> Amount { get; set; } 

    [DisplayName("Paid date")] 
    public System.DateTime PaidDate { get; set; } 


    public virtual Account Account { get; set; } 
    public virtual Client Client { get; set; } 
} 

Answer 1

可能是，它是MySQL的提供者（SQL表達式的創造者）的錯誤。但是，你有沒有嘗試過這種方式？

List<Payments> payments = db.Payments 
    .Where(e => e.Account.Active == true) 
    .GroupBy(e => e.IdAccount) 
    .Select(e => e.OrderBy(x => x.PaidDate)) 
    .Take(4) 
    .SelectMany(e => e).ToList(); 

如果不幫你，你可以隨時通過所有查詢到這樣的代碼：

List<Payments> payments = db.Payments 
    .Where(e => e.Account.Active == true) 
    .GroupBy(e => e.IdAccount) 
    .ToList() // From here all calculations will be on web server, not in SQL. So sql query will be simlier 
    .Select(e => e.OrderBy(x => x.PaidDate)) 
    .Take(4) 
    .SelectMany(e => e).ToList(); 

但是，它會影響到更流暢。