LINQ to Entities 选择多对多关系中的所有条目

发布于 2024-12-10 10:50:53 字数 515 浏览 0 评论 0原文

我有 3 个 MySql 表：Students、Classes 和 StudentsInClasses。

实体框架将它们转换为两个实体 Student 和 Class，每个实体都通过多对多导航属性链接到另一个实体（例如 Student.Classes代码>）。

但是没有 StudentsInClasses 实体，因此使用 LINQ to Entities（相当于 SQL）的最佳调用方式是什么：

SELECT StudentId, ClassId FROM StudentsInClasses;

我正在寻找 HashSet（或等效项） { StudentId, ClassId } 对，这样我就可以快速查找给定学生是否在给定班级中。（存储这个的最佳方式是什么？）

非常感谢。

原文

I have 3 MySql tables: Students, Classes and StudentsInClasses.

The Entity Framework translates these into two entities Student and Class, each linking to the other with a many-to-many navigation property (e.g. Student.Classes).

But there is no StudentsInClasses entity, so what's the best way to call, using LINQ to Entities, the equivalent of SQL:

SELECT StudentId, ClassId FROM StudentsInClasses;

I'm looking for a HashSet (or equivalent) of { StudentId, ClassId } pairs so I can quickly look up whether a given student is in a given class. (What's the best way of storing this?)

Many thanks.

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悍妇囚夫 2024-12-17 10:50:53

怎么样：（

var query = from @class in db.Classes
            from student in @class.Students
            select new { ClassId = @class.ID, Student = student };

var lookup = query.ToLookup(x => x.ClassId,
                            x => x.Student);

我怀疑这里的 Lookup 比 HashSet 更合适。）

编辑：如果您不想使用查询表达式：

var query = db.Classes
              .SelectMany(@class => @class.Students,
                          (@class, student) => new { ClassId = @class.ID,
                                                     Student = student });

How about:

var query = from @class in db.Classes
            from student in @class.Students
            select new { ClassId = @class.ID, Student = student };

var lookup = query.ToLookup(x => x.ClassId,
                            x => x.Student);

(I suspect a Lookup is more appropriate than a HashSet here.)

EDIT: If you don't want to use query expressions:

var query = db.Classes
              .SelectMany(@class => @class.Students,
                          (@class, student) => new { ClassId = @class.ID,
                                                     Student = student });

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