c# – EF Core,投影子集合渴望加载

发布时间：2020-12-15 07:43:38 所属栏目：百科来源：网络整理

导读：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,对于B集合中的2个项目. [a].[Id]

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,对于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]',@_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BName？

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：https://gist.github.com/lukemcgregor/692834629da09b21d5a35515e86c9002

解决方法

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()
    }))

见New features in EF Core 2.1中的Optimization of correlated subqueries.

（编辑：李大同）

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