Question

我的课程设计如下：

public class Video {
    public int Id {get; set;}
    public string Title {get; set;}

    public int PlaylistId {get; set;}
    [ForeignKey("PlaylistId")]
    public Playlist Playlist {get; set;}
}

public class Playlist {
    public int Id {get; set;}
    public string Description {get; set;}

    public List<Video> Videos {get; set;}

    public Playlist() {
        Videos = new List<Video>();
    }
}

现在，让我们创建一些对象：

Video video = new Video();
video.Title = "Titanic";

Playlist playlist = new Playlist();
playlist.Description = "Best videos";
playlist.Videos.Add(video);

context.Playlists.Add(playlist);
context.SaveChanges();

问题是我无法从我的数据库上下文中获取video.Playlist引用。当然，我可以添加一个获取所需属性的方法：

public static Playlist GetPlaylist(int videoId) {
    using (var context = new DatabaseContext()) {
        Video video = context.Videos.FirstOrDefault(x => x.Id == videoId);
        return context.Playlists.FirstOrDefault(x => x.Id == video.PlaylistId);
    }
}

但我认为必须有更好的解决方案。你能帮帮我吗？

Answer 1

尝试更正您的类声明（virtual和ICollection）。同时确保context.Configuration.LazyLoadingEnabled = true;：

public class Video 
{
    public int Id {get; set;}
    public string Title {get; set;}

    public int PlaylistId {get; set;}
    [ForeignKey("PlaylistId")]
    public virtual Playlist Playlist {get; set;}
}

public class Playlist 
{
    public int Id {get; set;}
    public string Description {get; set;}

    public virtual ICollection<Video> Videos {get; set;}
}

Answer 2

如果您想选择特定的东西，您可以选择它们：

var playlist = context.Videos
                   .Where(x => x.Id == videoId)
                   .Select(x => x.Playlist)
                   .FirstOrDefault();

您还可以使用Include指定要加载的引用实体

var video = context.Videos
                   .Include(x => x.Playlist)
                   .FirstOrDefault(x => x.Id == videoId);
var playlist = video.Playlist;

当@SlavaUtesinov回答时，第三个选项是virtual和延迟加载。但是如果你事先知道你想要播放列表，就不需要延迟加载，它只会创建不必要的中间查询。

如果您想要加载视频列表并且只想访问某些视频的播放列表，而不是大多数/所有视频，则延迟加载将是一种很好的方式。

获取物业的财产

2 个答案: