InvalidOperationException：尝试将实体的状态更改为“已修改”时，该属性具有临时值

Question

我有两班一对一的关系

public class Foo
{
    public int FooId { get; set; }

    public string Description { get; set; }

    public ICollection<Bar> Bars { get; set; }
}

public class Bar
{
    public int FooId { get; set; }
    public Foo Foo { get; set; }

    public int BarId { get; set; }

    public string Description { get; set; }
}

我想编辑一个Foo条目并需要添加新的Bar

var foo = _context.Foos.Include(b => b.Bars).FirstOrDefault(x => x.FooId == 8);
foo.Description = "changed";
_context.Entry(foo).State = EntityState.Modified;
foo.Bars.Add(new Bar() { Description = "new one",FooId=8,Foo=foo});
foreach (var b in foo.Bars)
        _context.Entry(b).State = b.BarId == 0 ? EntityState.Added : EntityState.Modified;
_context.SaveChanges();
return View();

我得到错误：

InvalidOperationException：在尝试将实体的状态更改为“已修改”时，实体类型“ Bar”的属性“ BarId”具有临时值。显式设置永久值，或确保将数据库配置为为此属性生成值