EF：包含 where 子句，+ SubIncludes

Question

这是对此处问题的跟进：Include with where clause。这个问题想找到所有驾驶公共汽车的清醒乘客

没有关于乘客的 WHERE 子句，这很简单，就像这样：

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

如果没有关于乘客的 WHERE 子句，包含来自乘客的子关系也很简单，如下所示：

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

但是这个问题需要在导航属性上使用 WHERE 子句。 前一个问题的答案非常完美，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，您如何使用此方法还包括乘客的随身行李和行李？乘客不可查询，因此此时您不能Include。我尝试过这样的事情，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 的极端到单个查询的极端。你曾经有很多查询，现在你想要一个，但请考虑一下幕后发生的事情。要获取数据实体框架需要交叉连接所有实体，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连接。

假设您有 5 辆正在行驶的巴士，每辆巴士有 30 名醒着的乘客和 15 件行李，因此您将获得巴士 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来查询乘客和行李，您将拥有更少的记录（5 * 30 + 5 * 15 = 225）并且每个实体将被提取一次。

执行一个返回所有内容的大查询并不是一个好主意——它更慢、更难维护并且不值得你花时间。只查询醒着的乘客，然后查询行李。

Answer 2

对于答案，向下滚动到答案部分。

免责声明：我爱英孚。对于在我的系统中进行的 99.999% 的调用，我可以最快地编写代码 (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）具有比手写 SQL 更快的执行计划。但这里不是这样。

研究科

顺便说一句：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 要为这些结果生成某些内容，则需要嵌套和关键字段才能知道如何映射它们。没什么大不了的。

不幸的是，为了通过一次访问数据库来实现这一点，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 语句有几百行。我破解了它，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，My Hand-Written SQL Query 返回 54 行，EF Generated Query 返回大约 30,000 行。因此，如果您只考虑数据的 Over-The-Wire 传输时间的增加，这是不可接受的。

回答部分

答案是：您可以使用 Linq to Entities（在 DB 上）和 Linq to Objects（在代码中）在一次调用中实现您的结果，但它不会高效。您可以改为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代码更易于理解。

最好的办法是执行多个查询。我就是这样做的：

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。 SQL 总共有大约 40 行。我破解了它，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 次往返调用中总共返回大约 100 行。所以这意味着对数据库的 4 次调用，但都非常小、可读且非常快速。

我没有计时，但是每当我在此答案代码上方的断点处暂停，然后按 F5 到结果的另一侧，它就是即时的。当我在研究中为 Single-Call 做同样的事情时，它花了整整一秒或更长时间，明显滞后。