在多个表之间搜索功能

Question

I'm new to ASP.NET MVC C# with EF6 in SQL Server, I have 3 tables:

1. Student Info
2. Courses
3. Stud_Courses to mapping between students and courses

Sample data:

Cou_ID | Stud_ID
-------+--------
     1 | 1
     1 | 2

I want to do searching in student page by student name or by course name for example if a user type math it will represents all students takes math I wrote this code that search by name correctly the problem is search by course name show error at s.Stud_ID in this line

StuList = db.Students.Where(x => x.StudentId == s.Stud_ID).ToList();

(Note : Stud_ID type is varchar in the database)

Stud_Courses does not contain a definition for 'Stud_ID' and no extension method 'Stud_ID' accepting a first argument of type 'List' could be found (are you missing a using directive or an assembly reference

My controller code

public JsonResult GetSearchingData(string SearchBy, string SearchValue){

List<Students> StuList = new List<Students>();

if (SearchBy == "Name")    
{
    StuList = db.Students.Where(x => x.StudentsName.Contains(SearchValue) || SearchValue == null).ToList();
    var subCategoryToReturn = SupList.Select(S => new { supervisorName = S.supervisorName });
    return Json(SupList, JsonRequestBehavior.AllowGet);   
}

if (SearchBy == "Course Name")
{
    var c = db.Courses.Where(x => x.CoursesName == SearchValue).SingleOrDefault();
    var CN = c.CourseId;
    var s = db.Stud_Courses.Where(x => x.Cou_ID == CN).ToList();
    StuList = db.Students.Where(x => x.StudentId == s.Stud_ID).ToList();  
    // here the error happens
    return Json(SupList, JsonRequestBehavior.AllowGet);}
}

Stud_Courses model

namespace MyModel.Models{
using System;
using System.Collections.Generic;

public partial class Stud_Courses{
    public int Id { get; set; }
    public Nullable<int> Cou_ID { get; set; }
    public string Stud_Id { get; set; }

    public virtual Courses Courses{ get; set; }
    public virtual Student Student { get; set; }
}}

Answer 1

"s" is a List so you need to select the Stud_Id, so s will be a List. Then you can get all students that are contained in s.

var s = db.Stud_Courses.Where(x => x.Cou_ID == CN).Select(x => x.Stud_Id).ToList();
StuList = db.Students.Where(x => s.Contains(x.StudentId)).ToList();

or you can reduce the 3 separate queries to

StuList = db.Students.Where(x => x.Stud_Courses.Any(y => y.Courses.CoursesName == SearchValue)).ToList();