EF核心“分组依据无法翻译，将在本地进行评估。”

Question

我正在使用Entity Framework Core 2.2.6对Sql Server数据库运行一个简单查询，但是GroupBy不在服务器上执行，而是在本地执行。

我有什么遗漏的东西会迫使该组进入服务器吗？

我试过的EF查询的2种变化：

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());

两种变体均产生以下SQL：

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

警告产生：

警告：Microsoft.EntityFrameworkCore.Query [20500] LINQ表达式'GroupBy（[h] .Date，[h]）'无法翻译，将在本地进行评估。

意见建议：

//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

这是因为没有这样的SQL查询。

像SQL一样思考。 如果要按日期组获取用户名，则两者都需要。

基本上：

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

这将产生这样的SQL查询。

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

之后，您可以根据需要使用数据创建字典的结构。

Answer 2

问题是，当您尝试对数据库进行分组时，实际上并没有实现组内值的方法。 您只能进入SELECT分组列或非分组列的汇总值（通过SUM等）。

例如：

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

该查询将产生结果集，其中每行将有两列，即日期和名称。

让我们尝试分组：

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

完全不会评估此SQL查询，因为从SQL Server角度来看该查询无效。 错误消息将是

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

总结一下，您可以按照@Ercan Tirman的建议进行操作，或者加载所有用户名和日期并在内存中将它们分组：

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());