高级投影查询

Question

任何更好的方法来做到这一点

using (var db = new SmartContext())
{
    var MyQuery = from idr in db.ID_SB
                  join tk in db.Track_SB on idr.MB_Track_ID equals tk.MB_TrackID
                  join talr in db.Track_Album_Reln on tk.MB_TrackID equals talr.MB_Track_ID
                  join tal in db.Album_SB on talr.MB_AlbumID equals tal.MB_Release_ID
                  orderby idr.Last_played descending
                  select new
                  {
                      mb = tk.MB_TrackID,
                      Hash = idr.Hash,
                      Title = tk.Title,
                      Album = tal.Album_Name,

                      Times_Played = idr.Times_played,
                      Last_Played = idr.Last_played

                  };

    string artist = "";
    var list = new[] { new {  Hash = "", Title = "", Album = "", Artist = "", Times_Played = "", Last_Played = ""}}.ToList();
    list.Clear();
    foreach (var q in MyQuery)
    {
        int i = 0;
        var art = db.Track_Artist_Reln.Where(a => a.MB_Track_ID == q.mb);
        foreach (var a in art)
        {
            var tart = db.Artist_SB.Where(ar => ar.MB_Artist_ID == a.MB_ArtistID).Select(ar => ar.Artist_Name);
            foreach (var tar in tart)
            { if (i != 0) { artist = artist + ", " + tar; } else { artist = tar; i++; } }
        }
        list.Add(new
        {
            Hash = q.Hash.ToString(),
            Title = q.Title.ToString(),
            Album = q.Album.ToString(),
            Artist = artist.ToString(),
            Times_Played = q.Times_Played.ToString(),
            Last_Played = q.Last_Played.ToString(),            
          });
    }


    ListView1.ItemsSource = list;
}

有没有更好的方法,因为它耗费了大量的时间

此代码用于此目的

在这我想要检索

• 哈希
• 标题
• ALBUMNAME
• 艺术家(包含单行艺术家中该歌曲的所有艺术家)
• 时代玩了
• 上次播放

通过加入除艺术家名称之外的所有东西,我能够获得的一切

有很多可能的关系主表ID_SB,Track_SB,Album_SB,Artist_SB其他是存储两个表的主键的关系

Answer 1

内部循环可以用 usingstring.Join(\xe2\x80\xa6)和来替换.SelectMany()，将嵌套列表 (really IEnumerable) 扁平化为一个：

\n\n

var artist = string.Join(", ",\n                         db.Track_Artist_Reln\n                           .Where(a => a.MB_Track_ID == q.mb)\n                           .SelectMany(a => db.Artist_SB\n                                              .Where(ar => ar.MB_Artist_ID == a.MB_ArtistID)\n                                              .Select(ar => ar.Artist_Name)));\n

\n\n

您可以使用以下命令，而不是手动将枚举转换为列表.ToList()：

\n\n

var list = MyQuery.Select(q => new{\n        Hash = q.Hash.ToString(),\n        Title = q.Title.ToString(),\n        Album = q.Album.ToString(),\n        Artist = string.Join(", ",\n                             db.Track_Artist_Reln\n                               .Where(a => a.MB_Track_ID == q.mb)\n                               .SelectMany(a => db.Artist_SB\n                                                  .Where(ar => ar.MB_Artist_ID == a.MB_ArtistID)\n                                                  .Select(ar => ar.Artist_Name))),\n        Times_Played = q.Times_Played.ToString(),\n        Last_Played = q.Last_Played.ToString(),            \n        }.ToList();\n

\n