EF核心“分組依據無法翻譯，將在本地進行評估。”

Question

我正在使用Entity Framework Core 2.2.6對Sql Server數據庫運行一個簡單查詢，但是GroupBy不在服務器上執行，而是在本地執行。

我有什么遺漏的東西會迫使該組進入服務器嗎？

我試過的EF查詢的2種變化：

public class Holiday
{
    public int Id {get;set;}
    public DateTime Date {get;set;}
    public string Username {get;set;}
    public string Approver {get;set;}
}

//version 1
await _dbContext.Holidays
    .GroupBy(h => new { h.Date})
    .ToDictionaryAsync(x => x.Key.Date, x => x.Select(x1 => x1.Username).ToList());

//version 2
await _dbContext.Holidays
    .GroupBy(h => h.Date)
    .ToDictionaryAsync(x => x.Key, x => x.Select(x1 => x1.Username).ToList());

兩種變體均產生以下SQL：

SELECT [h].[Id], [h].[Approver], [h].[Date], [h].[HolidayTypeId], [h].[OwningRequestId], [h].[HolidayStatusId], [h].[Username]
FROM [Holidays] AS [h]
ORDER BY [h].[Date]

警告產生：

警告：Microsoft.EntityFrameworkCore.Query [20500] LINQ表達式'GroupBy（[h] .Date，[h]）'無法翻譯，將在本地進行評估。

意見建議：

//group by string
await _dbContext.Holidays
     .GroupBy(h => h.Username)
     .ToDictionaryAsync(x => x.Key, x => x.Select(x1 => x1.Username).ToList());

//group by part of date
await _dbContext.Holidays
     .GroupBy(h => h.Date.Year)
     .ToDictionaryAsync(x => x.Key, x => x.Select(x1 => x1.Username).ToList());

--group by string
SELECT [h].[Id], [h].[Approver], [h].[Date], [h].[HolidayTypeId], [h].[OwningRequestId], [h].[HolidayStatusId], [h].[Username]
FROM [Holidays] AS [h]
ORDER BY [h].[Username]

--group by part of date
SELECT [h].[Id], [h].[Approver], [h].[Date], [h].[HolidayTypeId], [h].[OwningRequestId], [h].[HolidayStatusId], [h].[Username]
FROM [Holidays] AS [h]
ORDER BY DATEPART(year, [h].[Date])

Answer 1

這是因為沒有這樣的SQL查詢。

像SQL一樣思考。 如果要按日期組獲取用戶名，則兩者都需要。

基本上：

await _dbContext.Holidays
    .GroupBy(h => new { h.Date, h.Username})
    .Select(g => new
        {
          g.Key.Date,
          g.Key.Username
        });

這將產生這樣的SQL查詢。

SELECT [h].[Date],[h].[Username]
FROM [Holidays] AS [h]
GROUP BY [h].[Date],[h].[Username]

之后，您可以根據需要使用數據創建字典的結構。

Answer 2

問題是，當您嘗試對數據庫進行分組時，實際上並沒有實現組內值的方法。 您只能進入SELECT分組列或非分組列的匯總值（通過SUM等）。

例如：

SELECT [h].[Date], [h].[Username]
FROM [Holidays] AS [h]

該查詢將產生結果集，其中每行將有兩列，即日期和名稱。

讓我們嘗試分組：

SELECT [h].[Date], [h].[Username]
FROM [Holidays] AS [h]
GROUP BY [h.Date]

完全不會評估此SQL查詢，因為從SQL Server角度來看該查詢無效。 錯誤消息將是

Column 'Holidays.Username' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

總結一下，您可以按照@Ercan Tirman的建議進行操作，或者加載所有用戶名和日期並在內存中將它們分組：

var dateAndUsername = await _dbContext.Holidays
    .Select(x => new {x.Date, x.Username})
    .ToArrayAsync();

Dictionary<DateTime, List<string>> grouped = dateAndUsername
    .GroupBy(x => x.Date)
    .ToDictionary(x => x.Key, x => x.Select(y => y.Username).ToList());