EF Core，投影子集合渴望加载

Question

在EF核心中，我试图用子集合来计划结果集。

 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，它会触发每行的单独查询。 例如，B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，相同的查询会产生以下结果（正如我所料）：

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热切地加载子集合BNames ？

---

注意：这与包含.Include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导致错误。

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

Answer 1

这在EF Core 2.1中可以使用其相关的子查询优化。

您需要更新到EF Core 2.1并进行小修改以通过向子查询添加.ToList()来选择优化：

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

请参阅EF Core 2.1中新功能 的相关子查询优化 。