在实体框架中添加多对多对象C＃

Question

我使用EF和新的它。我有很多很多我的项目

假设我有3个表

• 类：的ClassID，类名
• 学生：StudentID-姓
• StudentClass：StudentID-的ClassID

如果我想将学生添加到我的班级，是否有必要为每个学生传递所有数据？

事情是这样的：

学生表：

 StudentID Name 
    ---------------------------- 
    1   Andrew 
    2   James 

类表：

 ClassID  Name 
    ---------------------------- 
    1   Math 

我的代码：

using (var db = factory.CreateContext()) 
{ 
    Class a = db.Classes.Where(x => x.ClassID == 1) 

    // Here if I create a `Student`, will it be stored in many to 
    // many table only passing `StudentID` ? 
    a.Students.Add(new Student { StudentID = 1 }) 
    a.Students.Add(new Student { StudentID = 2 }) 

    // or do I need do this? 
    // a.Students.Add(db.Students.where(x=> x.StudentID == 1))? 
} 

Answer 1

如果你已经在数据库中的学生，我强烈建议要做到这一点

// Find the class 
Class a = db.Classes.FirtsOrDefault(x => x.ClassID == 1); 

// be sure it exists 
if(a != null) 
{ 
    // find the student 
    Student s = db.Students.FirtsOrDefault(x => x.StudentID == 1); 

    // Always check if the object exist on the database, 
    // the row could be removed from another part of your program. 
    // Otherwise you will get an error on Add() because object is null 
    if(s != null) 
    { 
     a.Students.Add(s); 
    } 
} 

如果您有多个学生加入，你可以搜索那些然后用AddRange方法。

// find all the student you want (use the search you need) 
Student students = db.Students.Where(x => x.StudentID == 1 && x.StudentID == 2 && x.StudentID == 3); 

if(students.Count > 0){ 
    a.Students.AddRange(students); 
}