EF 核心 "Group By could not be translated and will be evaluated locally."

Question

我是运行使用 Entity Framework Core 2.2.6 针对 Sql 服务器数据库的简单查询，但是 GroupBy 没有在服务器上执行，而是正在执行本地。

有没有我遗漏的东西会迫使群组进入服务器？

我尝试过的 2 种 EF 查询变体：

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());

两种变体都会产生以下 SQL：

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

产生警告：

警告警告：Microsoft.EntityFrameworkCore.Query[20500] LINQ 表达式 'GroupBy([h].Date, [h])' 无法翻译，将在本地求值。

来自评论的建议：

//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

因为没有SQL这样的查询。

像SQL一样思考。如果你想按日期组获取用户名，你需要这两个。

基本上：

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

这将生成这样的 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来创建字典的结构。

Answer 2

问题是，当您尝试在数据库中分组时，您实际上没有办法在组内具体化值。您只能获得 SELECT 分组列或非分组列的聚合值（通过 SUM 等）。

例如：

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

此查询将生成结果集，其中每一行都有两列，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]

这个 SQL 查询根本不会被评估，因为从 SQL 服务器的角度来看它是无效的。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.

综上所述，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对它们进行分组：

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());

EF 核心 "Group By could not be translated and will be evaluated locally."

EF Core "Group By could not be translated and will be evaluated locally."

c#

ef-core-2.2