无法更改关系，因为一个或多个外键属性不可为空

Question

我正在尝试添加一些值，并尝试从我的数据库中删除选定的值。

我使用的代码如下：

[HttpPost]
        public ActionResult SavePlaylist(List<ItemEditViewModel> content, long playlistid, List<long> deleted, string Title)
        {
            var playlist = db.Playlists.Include("PlaylistContents").FirstOrDefault(x => x.PlaylistId == playlistid);


            for (int i = 0; i < content.Count; i++)
            {
                var pc = new PlaylistContent();
                pc.Sequence = content[i].MetaID;
                playlist.PlaylistContents.Add(pc);
            }
            for (int i = 0; i < deleted.Count; i++)
            {
                long delid = deleted[i];
                ar remove = playlist.PlaylistContents.FirstOrDefault(x => x.PlaylistContentId.Equals(delid));
                playlist.PlaylistContents.Remove(remove);


            }
            db.SaveChanges();
            return JSON(playlist);

        }

值被成功添加，但在删除其中的值时，错误显示如下：：

 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.

我能做些什么来解决这个错误。

Answer 1

下面的代码从集合中移除该对象：

playlist.PlaylistContents.Remove(remove);

但是，当您调用SaveChanges时，它会失败，因为FK列不可为空。为什么？因为EF不会删除该行，而是将id值设置为零。您需要删除该行，为此，您可以执行以下操作：

db.PlaylistContents.Remove(remove); // this line removes the row from the database
playlist.PlaylistContents.Remove(remove); // this line removes the object form the collection
db.SaveChanges();

Answer 2

我用类似的方法解决了这个问题：

DataModel.PlayListContent.Remove(remove)

Answer 3

两个不是Optional的对象之间存在关系，您可以将Optional添加到映射中以允许设置外键的null值。

这个问题EF 4.1: Difference between .WithMany() and .WithOptional() ?将会有所帮助。