EF:包含 where 子句，+ 子包含

Question

这是这里问题的后续：包含 with where 子句。 这个问题想找到驾驶巴士上所有醒着的乘客

如果没有关于乘客的 WHERE 条款，这非常简单，如下所示：

var result = Context.Busses.Where(x => x.IsDriving)
.Include(x => x.Passengers);

如果没有关于乘客的 WHERE 条款，包含乘客的子关系也非常简单，如下所示：

var result = Context.Busses.Where(x => x.IsDriving)
.Include(x => x.Passengers.CarryOns)
.Include(x => x.Passengers.Luggage);

但该问题需要在导航属性上使用 WHERE 子句。 上一个问题的答案很完美，没有子关系：

var result = Context.Busses.Where(x => x.IsDriving)
.Select(bus => new {bus, Passengers = bus.Passengers.Where(x => x.Awake)})
.AsEnumerable().Select(x => x.bus).ToList();

但是，如何使用这种方法将乘客的随身行李也包括在内？ 乘客不可查询，因此此时无法Include。我尝试了这样的事情，但第一部分刚刚被第二部分覆盖：

var bussesQuery = Context.Busses.Where(x => x.IsDriving)
.Include(x => x.Passengers.CarryOns)
.Include(x => x.Passengers.Luggage);
// var check = bussesQuery.ToList(); 
// The Sub-Relationship data is included here, but the Passengers are not filtered.
var result = bussesQuery
.Select(bus => new {bus, Passengers = bus.Passengers.Where(x => x.Awake)})
.AsEnumerable().Select(x => x.bus).ToList();
// The Sub-Relationship data is missing, but the Passengers are filtered

Answer 1

通过查看您之前的查询，我看到您从 N+1 的极端变为单个查询的极端。你以前有很多疑问，现在你想要一个，但请考虑一下引擎盖下发生了什么。若要获取数据实体框架，需要交叉联接所有实体，因此对于每个包含的实体，您会在结果中获得额外的列，并且结果与该包含交叉联接。

假设您有 5 辆驾驶巴士、30 名醒着乘客和每辆巴士 15 件行李，因此您得到的巴士 x 行李 x 乘客 = 2250 条记录，每条记录都包含乘客和行李数据。如果您使用单独的查询查询乘客和行李，您将拥有更少的记录(5 * 30 + 5 * 15 = 225(，并且每个实体将被获取一次。

做一个大查询来返回所有内容并不是一个好主意 - 它更慢，更难维护，不值得你花时间。只需查询醒着的乘客，然后查询行李。

Answer 2

有关答案，请向下滚动到答案部分。

免责声明：我爱英孚。 对于系统中 99.999% 的调用，我可以以最快的速度编写代码 (LINQ(，而 OR-Mapping 是最快的系统。 此外，生成的查询(虽然看起来令人困惑(的执行计划比手写的SQL快得多。 但这里的情况并非如此。

研究科

首先是旁白：查看我的最终请求的原始 SQL 是这样的：

SELECT * FROM [Busses] [bus]
LEFT JOIN [Passengers] [passenger] ON [passenger].[BusID] = [bus].[BusID] AND [passenger].[Awake] <> 1
LEFT JOIN [CarryOns] [carryOn] ON [carryOn].[PassengerID] = [passenger].[PassengerID]
LEFT JOIN [Luggages] [luggage] ON [luggage].[PassengerID] = [passenger].[PassengerID]
WHERE [bus].[IsDriving] = 1

当然，如果 EF 要为这些结果生成一些内容，则需要嵌套和关键字段来知道如何映射它们。 没什么大不了的。

不幸的是，为了通过对数据库的单次点击来实现这一点，我必须执行以下操作：

var busses = context.Set<BusEntity>().Where(x => x.IsDriving);
var passengers = context.Set<PassengerEntity>().Where(x => x.Awake);
var carryOns = context.Set<CarryOnEntity>();
var luggages = context.Set<LuggageEntity>();
var passengerJoins = passengers.GroupJoin(
carryOns,
x => x.PassengerID,
y => y.PassengerID,
(x, y) => new { Passenger = x, CarryOns = y }
)
.SelectMany(
x => x.CarryOns.DefaultIfEmpty(),
(x, y) => new { Passenger = x.Passenger, CarryOns = x.CarryOns }
).GroupJoin(
luggages,
x => x.Passenger.PassengerID,
y => y.PassengerID,
(x, y) => new { Passenger = x.Passenger, CarryOns = x.CarryOns, Luggages = y }
)
.SelectMany(
x => x.Luggages.DefaultIfEmpty(),
(x, y) => new { Passenger = x.Passenger, CarryOns = x.CarryOns, Luggages = x.Luggages }
);
var bussesToPassengers = busses.GroupJoin(
passengerJoins,
x => x.BusID,
y => y.Passenger.BusID,
(x, y) => new { Bus = x, Passengers = y }
)
.SelectMany(
x => x.Passengers.DefaultIfEmpty(),
(x, y) => new { Bus = x.Bus, Passengers = x.Passengers }
)
.GroupBy(x => x.Bus);
var rez = bussesToPassengers.ToList()
.Select(x => x.First().Bus)
.ToList();

我不抱怨EF生成的SQL，但单个SQL语句只有几百行。 我对其进行了黑客攻击，删除了 SELECT 列，并更改了一些 ID 以匹配这个问题，它是这样的：

SELECT *
FROM ( SELECT *
FROM   (SELECT *
FROM ( SELECT DISTINCT *
FROM  [dbo].[Bus] AS [Extent1]
LEFT OUTER JOIN  (SELECT *
FROM    [dbo].[Passenger] AS [Extent2]
LEFT OUTER JOIN [dbo].[CarryOns] AS [Extent3] ON [Extent2].[PassengerId] = [Extent3].[PassengerId]
LEFT OUTER JOIN [dbo].[Luggages] AS [Extent4] ON [Extent2].[PassengerId] = [Extent4].[PassengerId]
WHERE [Extent1].[IsDriving] = 1
)  AS [Distinct1] ) AS [Project2]
OUTER APPLY  (SELECT *
FROM   (SELECT *
FROM  [dbo].[Bus] AS [Extent6]
LEFT OUTER JOIN  (SELECT *
FROM    [dbo].[Passenger] AS [Extent7]
LEFT OUTER JOIN [dbo].[CarryOns] AS [Extent8] ON [Extent7].[PassengerId] = [Extent8].[PassengerId]
LEFT OUTER JOIN [dbo].[Luggages] AS [Extent9] ON [Extent7].[PassengerId] = [Extent9].[PassengerId]
WHERE ([Extent6].[IsDriving] = 1) AND ([Project2].[BusId] = [Extent6].[BusId]) ) AS [Project3]
OUTER APPLY  (SELECT *
FROM     [dbo].[Passenger] AS [Extent11]
LEFT OUTER JOIN [dbo].[CarryOns] AS [Extent12] ON [Extent11].[PassengerId] = [Extent12].[PassengerId]
LEFT OUTER JOIN [dbo].[Luggages] AS [Extent13] ON [Extent11].[PassengerId] = [Extent13].[PassengerId]
LEFT OUTER JOIN [dbo].[CarryOns] AS [Extent15] ON [Extent11].[PassengerId] = [Extent15].[PassengerId]
WHERE ([Extent11].[IsAwake] = 1) AND ([Project3].[BusId] = [Extent11].[BusId])
UNION ALL
SELECT *
FROM     [dbo].[Passenger] AS [Extent16]
LEFT OUTER JOIN [dbo].[CarryOns] AS [Extent17] ON [Extent16].[PassengerId] = [Extent17].[PassengerId]
LEFT OUTER JOIN [dbo].[Luggages] AS [Extent18] ON [Extent16].[PassengerId] = [Extent18].[PassengerId]
WHERE ([Extent16].[IsAwake] = 1) AND ([Project3].[BusId] = [Extent16].[BusId])
)  AS [Project7]
ORDER BY ........................

对于我的个人测试数据，我的手写 SQL 查询返回 54 行，EF 生成的查询返回大约 30,000 行。 因此，如果您只考虑数据离线传输时间的增加，这是不可接受的。

答案部分

答案是：您可以使用 Linq to Entities(在 DB 上(和 Linq to Objects(在代码中(在单个调用中实现结果，但它不会提高性能。 相反，您可以选择性能更好的多个调用，包括通过网络传输的数据更少、生成的查询更具可读性以及更易于理解的代码。

最好的办法是执行多个查询。 这是我的做法：

var bus = context.Set<BusEntity>().Where(x => x.IsDriving).ToList();
var busIDs = bus.Select(x => x.BusID).ToList();
var passengers = context.Set<PassengerEntity>().Where(x => x.IsAwake && busIDs.Contains(x.BusID)).ToList();
var passengerIDs = passengers.Select(x => x.PassengerID).ToList();
var carryOns = context.Set<CarryOnEntity>().Where(x => passengerIDs.Contains(x.PassengerID)).ToList();
var luggages = context.Set<LuggageEntity>().Where(x => passengerIDs.Contains(x.PassengerID)).ToList();
passengers.ForEach(x => {
x.CarryOns = carryOns.Where(y => y.PassengerID == x.PassengerID).ToList();
x.Luggages = luggages.Where(y => y.PassengerID == x.PassengerID).ToList();
});
bus.ForEach(x => x.Passengers = passengers.Where(y => y.BusID == x.BusID).ToList());

这产生了 4 个调用。 总的来说，SQL大约有40行。 我对其进行了黑客攻击，删除了 SELECT 列，并更改了一些 ID 以匹配这个问题，它是这样的：

SELECT * FROM [dbo].[Busses] AS [Extent1]
WHERE [Extent1].[IsDriving] = 1
SELECT * FROM [dbo].[Passengers] AS [Extent1]
WHERE ([Extent1].[Awake] = 1) AND ([Extent1].[BusID] IN (......................))
SELECT * FROM [dbo].[CarryOns] AS [Extent1]
WHERE [Extent1].[PassengerID] IN (......................)
SELECT * FROM [dbo].[Luggages] AS [Extent1]
WHERE [Extent1].[PassengerID] IN (......................)

EF 生成的查询在 4 个往返调用中总共返回大约 100 行。这意味着对数据库的 4 次调用，但都非常小、可读且非常快。

我没有计时，但是每当我暂停在这个答案代码上方的断点上，以及结果另一边的 F5 时，它都是即时的。 当我在我的研究中对单次呼叫做同样的事情时，它花了一秒钟或更长时间，明显的滞后。