c# - 为什么要插入倍数？

Question

我有以下方法

public static Artist ProcessArtist(Artist artist, Entities db) {
    var artistLookup = db.Artist.SingleOrDefault(x => x.ExternalId == artist.ExternalId);
    if (artistLookup == null) {
        artistLookup = new Artist {
            ExternalId = artist.ExternalId,
            Name = artist.Name
        };
        db.Artist.AddObject(artistLookup);
        db.SaveChanges();
    }
    return artistLookup;
}

我使用它的方式是传入一个 Artist 对象，它只设置了 ExternalId 和 Name，但它不是完全连接到我的实体上下文。它是从外部来源抓取的。

当我调用 db.SaveChanges() 时，它会抛出错误，说我已经破坏了 ExternalId 的唯一键约束。我不知道它会如何尝试插入倍数。

有没有人有什么见解？谢谢!

编辑:我在下面添加了我的调用代码

var albums = from item in externalSource
             select new Album {
                 Country = country // Another entity, one that exists in the database
                 Name = item["Name"].Value,
                 Artist = new Artist {
                     ExternalId = Int32.Parse(item["ArtistId"].Value),
                     Name = item["ArtistName"].Value
                 }
             };

然后我为每张专辑调用 ProcessArtist

foreach (var album in albums) {
    album.Artist = ProcessArtist(album.Artist, db);
    db.Album.AddObject(album);
}

Answer 1

var artistLookup = db.Artist.SingleOrDefault(x => x.ExternalId == artist.ExternalId);
//in this line  you get an artist from the database


//i dont know why would you want to insert the artist that you just get from the database
//you are reinserting an existing record. That's the reason you get the error
    db.Artist.AddObject(artistLookup);
    db.SaveChanges();