Question

我使用EF和新手。我的项目中有很多对

假设我有3个表

类：ClassID-ClassName
学生：StudentID-FirstName
StudentClass：StudentID-ClassID

如果我想将学生添加到班级，是否需要传递每个学生的所有数据？

这样的事情：

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

如果您已经有数据库中的学生，我强烈建议您这样做

// 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);
    }
}

如果您要添加多个学生，可以搜索这些学生，然后使用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);
}

在Entity Framework C＃中添加多对多对象

1 个答案: