Entity Framework（05）：主从表的增删改

一、添加数据

1、在主表中添加从表数据

在景点的住宿集合（Lodgings）中增加一个度假区（Resort）

var dest = (from d in context.Destinations where d.Name == "Bali" select d).Single();

var resort = new CodeFirst.Model.Resort
{
    Name = "Pete's Luxury Resort",
};

dest.Lodgings.Add(resort);

context.SaveChanges();

2、添加主表的同时添加从表数据

添加一个带两个住宿的景点

var destination = new CodeFirst.Model.Destination
{
    Name = "AnHui HuangShan",
    Lodgings = new List
                    {
                        new CodeFirst.Model.Lodging {Name="HuangShan Hotel"},
                        new CodeFirst.Model.Lodging {Name="YingKeSong Hotel"}
                    }
};
context.Destinations.Add(destination);

context.SaveChanges();

3、添加从表的同时添加主表数据

添加一个带有景点信息度假村到住宿信息中。

var resort = new CodeFirst.Model.Resort
{
    Name = "Top Notch Resort and Spa",
    Destination = new CodeFirst.Model.Destination
    {
        Name = "Stowe, Vermont",
        Country = "USA"
    }
};

using (var context = new CodeFirst.DataAccess.BreakAwayContext())
{
    context.Lodgings.Add(resort);

    context.SaveChanges();
}

二、修改关联

1、修改从表的外键

var hotel = (from l in context.Lodgings where l.Name == "YingKeSong Hotel" select l).Single();
var reef = (from d in context.Destinations where d.Name == "Bali" select d).Single();

hotel.Destination = reef;
context.SaveChanges();

2、从表与主表脱离关系

1、ForeignKeys方式：

var davesDump = (from l in context.Lodgings where l.Name == "HuangShan Hotel" select l).Single();
davesDump.DestinationID = null;//(ForeignKeys方式）
context.SaveChanges();
  

2、Reference方式：

var davesDump = (from l in context.Lodgings where l.Name == "HuangShan Hotel" select l).Single();

context.Entry(davesDump).Reference(l => l.Destination).Load();  //找主表数据
davesDump.Destination = null;  //清空,(Reference方式）
context.SaveChanges();

三、删除关联数据

1、删除主表的同时删除相关联的从表数据（级联删除）

如果数据库里设置是级联删除，则不显示加载从表数据。

var canyon = (from d in context.Destinations where d.Name == "AnHui HuangShan" select d).Single();

context.Entry(canyon).Collection(d => d.Lodgings).Load();  //从表显示加载后，再删除主表数据
context.Destinations.Remove(canyon);
context.SaveChanges();

2、普通删除

删除主表数据，同时标注从表数据为删除状态（数据库关闭了级联删除的情况，可以手动去数据库的外键关系修改，也可以Fluent API配置关闭级联删除）

var canyon = (from d in context.Destinations where d.Name == "Grand Canyon" select d).Single();

foreach (var lodging in canyon.Lodgings.ToList())
{
    context.Lodgings.Remove(lodging);   //先标记相关的从表数据为删除状态
}
context.Destinations.Remove(canyon);    //再标记主表数据为删除装填
context.SaveChanges();   //执行上面的所有标记