在实体框架linq中两次包含相同字段时的行为

Question

当我两次包含同一个字段时会发生什么，这意味着我从db获取一个实体，并使用EF .include选项。我的意思是：

我有：

.Include(x => x.Student)
.Include(x => x.Car)
.Include(x => x.Student)

这就是模式：

Person has a Student
Person has a car

因此，如果(错误地)将学生包括两次(因为我的人只有一个学生)，有问题吗？

我只想把它包括一次！因为我只有一个学生。他抱怨这事吗？我试过了，看起来没问题，但我不知道这意味着什么。有人能解释/详细说明吗？搜索了一下，但找不到任何问题。

Answer 1

取此样本：

public class RentContext : DbContext
{
    public DbSet<Student> Students { get; set; }

    public DbSet<Rent> Rents { get; set; }

    public DbSet<Car> Cars { get; set; }
}

public class Car
{
    public int Id { get; set; }

    public string Model { get; set; }

    public double Price { get; set; }
}

public class Rent
{
    public int Id { get; set; }

    public Student Student { get; set; }

    public Car Car { get; set; }
}

public class Student
{
    public int Id { get; set; }

    public string Name { get; set; }

    public int Year { get; set; }
}

租金包括学生和汽车。

让我们使用唯一的包含子句进行查询：

var rents = ctx.Rents
    .Include(x => x.Student)
    .Include(x => x.Car)
    //.Include(x => x.Student)
    .ToList();

这是生成的sql：

SELECT
[Extent1].[Id] AS [Id],
[Extent2].[Id] AS [Id1],
[Extent2].[Name] AS [Name],
[Extent2].[Year] AS [Year],
[Extent3].[Id] AS [Id2],
[Extent3].[Model] AS [Model],
[Extent3].[Price] AS [Price]
FROM   [dbo].[Rents] AS [Extent1]
LEFT OUTER JOIN [dbo].[Students] AS [Extent2] ON [Extent1].[Student_Id] = [Extent2].[Id]
LEFT OUTER JOIN [dbo].[Cars] AS [Extent3] ON [Extent1].[Car_Id] = [Extent3].[Id]

让我们创建一个复制包含的查询：

var rents = ctx.Rents
    .Include(x => x.Student)
    .Include(x => x.Car)
    .Include(x => x.Student)
    .ToList();

您将得到以下sql：

SELECT
[Extent1].[Id] AS [Id],
[Extent2].[Id] AS [Id1],
[Extent2].[Name] AS [Name],
[Extent2].[Year] AS [Year],
[Extent3].[Id] AS [Id2],
[Extent3].[Model] AS [Model],
[Extent3].[Price] AS [Price]
FROM   [dbo].[Rents] AS [Extent1]
LEFT OUTER JOIN [dbo].[Students] AS [Extent2] ON [Extent1].[Student_Id] = [Extent2].[Id]
LEFT OUTER JOIN [dbo].[Cars] AS [Extent3] ON [Extent1].[Car_Id] = [Extent3].[Id]

如您所见，EF非常聪明，即使您不止一次指定包含，也可以生成相同的sql。

更新:重复包含(多次)

让我们试试这个：

var rents = ctx.Rents
    .Include(x => x.Student)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Car)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .Include(x => x.Student)
    .ToList();

重复包括和多次。下面是生成的sql：

SELECT
    [Extent1].[Id] AS [Id],
    [Extent2].[Id] AS [Id1],
    [Extent2].[Name] AS [Name],
    [Extent2].[Year] AS [Year],
    [Extent3].[Id] AS [Id2],
    [Extent3].[Model] AS [Model],
    [Extent3].[Price] AS [Price]
    FROM   [dbo].[Rents] AS [Extent1]
    LEFT OUTER JOIN [dbo].[Students] AS [Extent2] ON [Extent1].[Student_Id] = [Extent2].[Id]
    LEFT OUTER JOIN [dbo].[Cars] AS [Extent3] ON [Extent1].[Car_Id] = [Extent3].[Id]

又是同样的代码。所以，是的。我们可以说一切都会好起来的，尽管这是件很奇怪的事。

希望这能有所帮助！

Answer 2

如果您两次调用它，将生成相同的查询。结果一样，就像这样

SELECT
    [Extent1].[Id] AS [Id],
    [Extent1].[Title] AS [Title],
    [Extent1].[PersonId] AS [PersonId],
    [Extent2].[Id] AS [Id1],
    [Extent2].[Name] AS [Name]
    FROM  [dbo].[Books] AS [Extent1]
    INNER JOIN [dbo].[People] AS [Extent2] ON [Extent1].[PersonId] = [Extent2].[Id]

Answer 3

这是完全有效的，不会绊倒EF在任何情况下。考虑一下如何包含附加的引用和集合，这些引用和集合挂起像Student这样的引用。你可以这样写。您提出的建议不会比下面的示例更混淆EF。

.Include(x => x.Student.Teacher)
.Include(x => x.Car)
.Include(x => x.Student.DrivingLog)