EF Core，投影子集合急切加载

Question

在 EF 核心中，我试图用子集合投影出一个结果集。

 ctx.As.Select(a => new
    {
        AId = a.Id,
        BNames = a.Bs.Select(x=>x.Name) //SubCollection
    })

但是，当执行此操作时，BNames 集合是惰性的，因此当它被枚举时，它会为每一行触发一个单独的查询。例如对于 B 的集合中的 2 个项目。

[a].[Id] AS [AId] FROM [As] AS [a]

p_executesql N'SELECT [x].[Name]
FROM [Bs] AS [x]
WHERE @_outer_Id = [x].[AId]',N'@_outer_Id int',@_outer_Id=1

p_executesql N'SELECT [x].[Name]
FROM [Bs] AS [x]
WHERE @_outer_Id = [x].[AId]',N'@_outer_Id int',@_outer_Id=2

在 EF6 中，相同的查询结果如下（如我所料）：

SELECT 
[Extent1].[Id] AS [Id], 
[Project1].[C1] AS [C1], 
[Project1].[Name] AS [Name]
FROM  [dbo].[A] AS [Extent1]
LEFT OUTER JOIN  (SELECT 
    [Extent2].[Name] AS [Name], 
    [Extent2].[A_Id] AS [A_Id], 
    1 AS [C1]
    FROM [dbo].[B] AS [Extent2] ) AS [Project1] ON [Extent1].[Id] = [Project1].[A_Id]
ORDER BY [Extent1].[Id] ASC, [Project1].[C1] ASC

如何让 EF 急切加载子集合 BNames？

---

注意：这与包含 .Include 的子实体不同，因为这是自定义投影，而不是导航属性。在此处使用.Include 会导致错误。

完整的回购代码在这里：https://gist.github.com/lukemcgregor/692834629da09b21d5a35515e86c9002

Answer 1

这在 EF Core 2.1 中使用其相关子查询优化是可能的。

您需要更新到 EF Core 2.1 并通过在子查询中添加 .ToList() 进行小修改以选择优化：

foreach (var thing in ctx.As
    .Select(a => new
    {
        AId = a.Id,
        BNames = a.Bs.Select(x => x.Name).ToList()
    }))

在New features in EF Core 2.1 中查看Optimization of correlated subqueries。