新增关联对象

对于显式关联型，我们可以直接通过ObjectSet添加关联对象也可以通过为关联引用设值或添加元素的方式添加关联对象，后一种方式与隐式关联型并无区别，可以参考新增关联一节，此处仅介绍通过ObjectSet添加：

var context = new ClassAndTeacherContext();

//新增一个班级
var newclass = new Class
{
    Name = @"某某班"
};

//新两个增老师
var teacher1 = new Teacher {Name = "老师1"};
var teacher2 = new Teacher {Name = "老师2"};

//与第一个老师建立关联
var teaching1 = new Teaching
{
    Class = newclass,
    Teacher = teacher1,
    IsManage = true
};

//与第二个老师建立关联
var teaching2 = new Teaching
{
    Class = newclass,
    Teacher = teacher2,
    IsManage = false
};

//持久化
context.ClassTeachers.Attach(teaching1);
context.ClassTeachers.Attach(teaching2);
context.SaveChanges();