此服务方法将返回IQueryable<Vehicle>：
public IQueryable<Vehicle> GetVehicles()
{
return
from v in _context.Vehicles
where
v.Schedule == true
&& v.Suspend == false
select v;
}
如果我想在查询中包括另一个表：
public IQueryable<Vehicle> GetVehicles()
{
return
from v in _context.Vehicles
join si in _context.ServiceIntervals on v.Id equals si.VehicleId into loj
from rs in loj.DefaultIfEmpty()
where
v.Schedule == true
&& v.Suspend == false
select new { vehicle = v, repair = rs };
}
正确的IQueryable<T>返回类型是什么？ IQueryable<{vehicle,repair}>不正确。
是否有更好的方式编写select语句？
**编辑**
我希望保持简单，但我认为澄清是有用的。
ServiceIntervals实际上是一个IQueryable<T>，它引用了另一个SQL DB中的表值函数：
public IQueryable<ServiceInterval> ServiceIntervals(DateTime startingDate, DateTime endingDate) =>
Query<ServiceInterval>().FromSql($@"
SELECT *
FROM OtherDatabase.Dbo.ServiceIntervals({startingDate}, {endingDate})"
);
原始查询实际上包括日期：
...
_context.ServiceIntervals(DateTime.Now.Date,DateTime.Now.Date)
...
因此，我认为ServiceIntervals不能作为导航属性公开在Vehicle实体上。
答案 0 :(得分：3)
在LINQ中执行def process(context: Context, elements: Iterable[IN], out: Collector[OUT])会创建一个匿名类型，根据定义，该类型是匿名的，不能用作返回类型。如果要返回此变量，则必须为其创建一个类型。
select new {}您可以在自定义DTO类中更改类型和变量名称以匹配public class VehicleServiceDTO
{
public Vehicle Vehicle { get; set; }
public ServiceInterval Repair { get; set; }
}
public IQueryable<VehicleServiceDTO> GetVehicles()
{
return
from v in _context.Vehicles
join si in _context.ServiceIntervals on v.Id equals si.VehicleId into loj
from rs in loj.DefaultIfEmpty()
where
v.Schedule == true
&& v.Suspend == false
select new VehicleServiceDTO() { Vehicle = v, Repair = rs };
}
的类型（我假设它被称为_context.ServiceIntervals）。
答案 1 :(得分：2)
我不一定认为您不应该在这里使用导航属性。它们是虚拟的，并基于Linq语句生成查询。
在这种情况下，我也不认为单独的DTO模型有意义。由于您提到了导航属性，因此我假设您的关系是一对多的。我的猜测是，应该对车辆进行修理。如果真是这样，那么GroupJoin可能会起作用。 EF支持它：https://docs.microsoft.com/en-us/dotnet/framework/data/adonet/ef/language-reference/supported-and-unsupported-linq-methods-linq-to-entities
我添加了一个属性，用于维修车辆：
public IEnumerable<ServiceInterval> RepairsToSchedule { get; set; }
然后查询最终看起来像这样：
public static IQueryable<Vehicle> GetVehicles()
{
return _context.Vehicles
.Where(v => v.Schedule && !v.Suspend)
.GroupJoin(_context.ServiceIntervals,
v => v.Id,
si => si.VehicleId,
(v, si) => SetServiceIntervals(v, si));
}
我使用下面的静态方法将 ServiceIntervals 添加到车辆：
private static Vehicle SetServiceIntervals(Vehicle v, IEnumerable<ServiceInterval> si)
{
v.RepairsToSchedule = si;
return v;
}
整个源代码如下： 已编辑：针对两个单独的数据库进行了调整（请注意，在同一服务器上）
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Diagnostics;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
namespace StackoverFlow
{
class Program
{
private static FakeDatabaseContext _context = new FakeDatabaseContext();
private static FakeDatabaseContext2 _context2 = new FakeDatabaseContext2();
static void Main(string[] args)
{
CleanContext();
LoadContext();
foreach (var vehicle in GetVehicles())
{
Console.WriteLine(JsonConvert.SerializeObject(vehicle, Formatting.Indented));
Console.WriteLine();
}
Console.ReadKey();
}
public static IQueryable<Vehicle> GetVehicles()
{
return _context.Vehicles
.Where(v => v.Schedule && !v.Suspend)
.GroupJoin(_context.ServiceIntervals(new DateTime(), new DateTime()),
v => v.Id,
si => si.VehicleId,
(v, si) => SetServiceIntervals(v, si));
}
private static Vehicle SetServiceIntervals(Vehicle v, IEnumerable<ServiceInterval> si)
{
v.RepairsToSchedule = si;
return v;
}
#region EF Context
public class FakeDatabaseContext : DbContext
{
public DbSet<Vehicle> Vehicles { get; set; }
private DbSet<ServiceInterval> _serviceIntervals { get; set; }
public IQueryable<ServiceInterval> ServiceIntervals(DateTime startingDate, DateTime endingDate)
{
return _serviceIntervals.FromSql($@"
SELECT *
FROM Stackoverflow2.dbo.ServiceIntervals"
);
}
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(
@"Server=(localdb)\mssqllocaldb;Database=Stackoverflow;Integrated Security=True");
optionsBuilder
.ConfigureWarnings(w => w.Throw(RelationalEventId.QueryClientEvaluationWarning));
}
}
// Used to load a seperate database
public class FakeDatabaseContext2 : DbContext
{
public DbSet<ServiceInterval> ServiceIntervals { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(
@"Server=(localdb)\mssqllocaldb;Database=Stackoverflow2;Integrated Security=True");
optionsBuilder
.ConfigureWarnings(w => w.Throw(RelationalEventId.QueryClientEvaluationWarning));
}
}
public class Vehicle
{
public string Id { get; set; }
public bool Schedule { get; set; }
public bool Suspend { get; set; }
public IEnumerable<ServiceInterval> RepairsToSchedule { get; set; }
}
public class ServiceInterval
{
public string Id { get; set; }
public string VehicleId { get; set; }
}
#endregion EF Context
#region Seed methods
private static Random _random = new Random();
private static bool _randomBool => _random.Next() % 2 == 1;
private static void LoadContext()
{
var maxVehicles = 10;
for (int i = 1; i < maxVehicles; i++)
{
_context.Vehicles.Add(new Vehicle { Id = i.ToString(), Schedule = _randomBool, Suspend = _randomBool });
for (int o = 1; o < _random.Next(10); o++)
{
_context2.ServiceIntervals.Add(new ServiceInterval { Id = ((maxVehicles * i) + o).ToString(), VehicleId = i.ToString() });
}
};
_context.SaveChanges();
_context2.SaveChanges();
}
private static void CleanContext()
{
_context.Vehicles.RemoveRange(_context.Vehicles.ToArray());
_context2.ServiceIntervals.RemoveRange(_context2.ServiceIntervals.ToArray());
_context.SaveChanges();
_context2.SaveChanges();
}
#endregion Seed methods
}
}
IntelliTrace显示此查询已执行：
SELECT [v].[Id], [v].[Schedule], [v].[Suspend], [si].[Id], [si].[VehicleId]
FROM [Vehicles] AS [v]
LEFT JOIN (
SELECT *
FROM Stackoverflow2.dbo.ServiceIntervals
) AS [si] ON [v].[Id] = [si].[VehicleId]
WHERE ([v].[Schedule] = 1) AND ([v].[Suspend] = 0)
ORDER BY [v].[Id]
我对此进行了几次测试，一切似乎都正常。
〜干杯
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