在EF中没有联接表的情况下，如何在具有多对多关系的两个表（实体）之间使用LINQ进行JOIN查询？

Question

I didn't explicitly defined the composite table, as EF handle it automatically, but how can query results from one table with the other's Id: take this example:

public class Student
{
public int Id {get; set;}
public string Name{get; set;}
public ICollection<Course> Courses {get; set;}
}

public class Course
{
public int Id {get; set;}
public string Name {get; set;}
public ICollection<Student> Students {get; set;}
}

How I can get all courses for the Student with id = 3 I made this query:

var studentCourses = from s in context.Students join c in context.Courses on s.Id equals ?? what?

Should I define the joining table?

Answer 1

// Return all courses where at least one student has a specific id.
var studentId = 3;
var courses = context.Courses.Where(x => x.Students.Any(y => y.Id == studentId));

Or

var studentId = 3;
var courses = context.Students.Where(x => x.Id == studentId).SelectMany(x => x.Courses);

Should I define the joining table?

No, unless it contains anything of interest. Does it add any more information than the connection between the tables, such as DateAdded ? In that case I would create an entity for the table. But not otherwise.

Answer 2

Why not use property Courses from the Student class:

List<Course> coursesOfStudent3;
using (var dbContext = new MySchoolDbContext())
{
    coursesOfStudent3 = dbContext.Students
        .Where(student => student.studentId == 3)
        .Single()
        .Courses
        .ToList();
}

You get a problem if there is no student with StudentId == 3. Consider using SelectMany:

coursesOfStudent3 = dbContext.Students
    .Where(student => student.studentId == 3)
    .SelectMany(student => student.Courses)
    .ToList();

This alternative works if there is no student with StudentId == 3. However it only works if you can guarantee that your Where clause returns only one student, which in case of selection by ID is of course the case.