c# – 为什么EntityCollection.Remove()会导致SaveChanges()引发

CREATE TABLE master (
    id integer primary key autoincrement,title text);
CREATE TABLE slave (
    id integer primary key autoincrement,master_id integer not null,title text,foreign key (master_id) references master (id));

我创建了一个sqlite数据库,然后我用它来创建一个Entity Framework .edmx文件.

然后我添加一个主记录和一个奴隶记录,工作正常,然后我尝试删除从记录,但这引发了一个异常,我不知道为什么.

var ctx = new blaEntities();
var newMaster = master.Createmaster(0);
ctx.masters.AddObject(newMaster);

var newSlave = slave.Createslave(0,0);
newMaster.slaves.Add(newSlave);

ctx.SaveChanges(); // works fine,both id and master_id properties of newMaster 
                   // and newSlave are then set with generated values.

newMaster.slaves.Remove(newSlave);

ctx.SaveChanges(); // InvalidOperationException is raised

异常的消息是：

The operation failed: The relationship could not be changed because
one or more of the foreign-key properties is non-nullable. When a
change is made to a relationship,the related foreign-key property is
set to a null value. If the foreign-key does not support null values,
a new relationship must be defined,the foreign-key property must be
assigned another non-null value,or the unrelated object must be
deleted.

我究竟做错了什么？

编辑：

这是由EF Createslave(…)方法生成的：

public static slave Createslave(global::System.Int64 id,global::System.Int64 master_id)
{
    slave slave = new slave();
    slave.id = id;
    slave.master_id = master_id;
    return slave;
}