如何使此LINQ查詢更整潔？

Question

我最近寫了一個LINQ查詢來獲取包含最近6個月的刊登位置數量的Dictionary 。

它返回一個月Dictionary字符串-小數對。

似乎有點笨拙。 你們中的LINQ大師中有誰能夠幫助我進行重構以使其更加整潔？

/// <summary>
/// Gets the last 6 months of Placement History totalled by Month 
/// for all Agencies
/// </summary>
/// <returns></returns>
public Dictionary<string, decimal> getRecentPlacementHistory()
{
    var placementHistoryByMonth = new Dictionary<string, decimal>();

    using (DemoLinqDataContext db = new DemoLinqDataContext())
    {
        for (int i = 0; i < 6; i++)
        {
            Decimal monthTotal = 
              (from a in db.Accounts
               where 
                 (a.Date_Assigned.Value.Month == DateTime.Now.AddMonths(-i).Month &&
                  a.Date_Assigned.Value.Year == DateTime.Now.AddMonths(-i).Month)
               select a.Amount_Assigned).Sum();
            String currentMonth = DateTime.Now.AddMonths(-i).ToString("MMM");

            placementHistoryByMonth.Add(currentMonth, monthTotal);
        }
        return placementHistoryByMonth;
    }
}

Answer 1

第一個問題：

where (a.Date_Assigned.Value.Month == DateTime.Now.AddMonths(-i).Month &&
       a.Date_Assigned.Value.Year == DateTime.Now.AddMonths(-i).Month)

后一個表達式不應該以.Year而不是.Mon結尾嗎？ 當然，您很少會得到價值1-12的年份...

由於您經常使用“當前月份”，因此我將其提取出來。 請注意，您還會多次使用當前時間，如果該時間在月底的午夜運行，則可能會產生奇怪的結果...

public Dictionary<string, decimal> getRecentPlacementHistory()
{
    var placementHistoryByMonth = new Dictionary<string, decimal>();
    using (DemoLinqDataContext db = new DemoLinqDataContext())
    {
        DateTime now = DateTime.Now;

        for (int i = 0; i < 6; i++)
        {
            DateTime selectedDate = now.AddMonths(-i);

            Decimal monthTotal = 
               (from a in db.Accounts
                where (a.Date_Assigned.Value.Month == selectedDate.Month &&
                       a.Date_Assigned.Value.Year == selectedDate.Year)
                select a.Amount_Assigned).Sum();

            placementHistoryByMonth.Add(selectedDate.ToString("MMM"),
                                        monthTotal);
        }
        return placementHistoryByMonth;
    }
}

我意識到這可能是您要擺脫的循環。 您可以嘗試計算整個批次日期的上下限，然后在相關范圍內按a.Date_Assigned的年/月分組。 老實說，這不會太漂亮。 請注意，如果可以進行查詢，那只會是對數據庫的一個查詢。

Answer 2

使用分組依據

DateTime now = DateTime.Now;
DateTime thisMonth = new DateTime(now.Year, now.Month, 1);

Dictionary<string, decimal> dict;
using (DemoLinqDataContext db = new DemoLinqDataContext())
{
    var monthlyTotal = from a in db.Accounts
        where a.Date_Assigned > thisMonth.AddMonths(-6)
        group a by new {a.Date_Assigned.Year, a.Date_Assigned.Month} into g
        select new {Month = new DateTime(g.Key.Year, g.Key.Month, 1),
                    Total = g.Sum(a=>a.Amount_Assigned)};

    dict = monthlyTotal.OrderBy(p => p.Month).ToDictionary(n => n.Month.ToString("MMM"), n => n.Total);
}

無需循環！

Answer 3

如果您不擔心缺少數據的月份，那么我在執行以下操作時也會遇到類似的問題：（轉換為您的變量）

  DateTime startPeriod = 
     new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1).AddMonths(-6);

  var query1 = from a in db.Accounts where a.Date_Assigned >= startPeriod
 group a by new { a.Date_Assigned.Year  ,a.Date_Assigned.Month  } into result
 select new
 {
     dt = new DateTime( result.Key.Year, result.Key.Month , 1),
     MonthTotal = result.Sum(i => i.Amount_Assigned)
 } ;             

  var dict = query1.OrderBy(p=> p.dt).ToDictionary(n => n.Dt.ToString("MMM") , n => n.MonthTotal );