EntityCommandExecutionException - 尝试在 1 个数据库中链接 2 个模型 [重复]

Question

我几乎有 2 个模型 - 一个公告，有些人可以发布公告和一个看到的模型，它决定是否有人看到了公告。她是模特：

宣布：

public class Announcement
{
    public int AnnouncementId { get; set; }
    public string AnnouncementContent { get; set; }
    public virtual ApplicationUser User { get; set; }
}

看到了：

public class Seen
{
    public int SeenId { get; set; }

    public virtual Announcement Announcement { get; set; }
    public virtual ApplicationUser User { get; set; }
}

在我的 AnnouncementController.Index 中，我有这段代码，如果您查看此页面，请将每个公告标记为已看到但在“新看到”部分出现错误：

public ActionResult Index()
{
    string currentUserId = User.Identity.GetUserId();
    var currentUser = db.Users.FirstOrDefault(x => x.Id == currentUserId);


    if(db.Announcements != null)
    {
        foreach (Announcement anoun in db.Announcements)
        {
            new Seen
            {
                User = db.Users.Add(currentUser),
                Announcement = db.Announcements.FirstOrDefault(x => x.AnnouncementId == anoun.AnnouncementId),
            };
        }
    }
    return View(db.Announcements.ToList());
}

已经有一个打开的 DataReader 与此命令关联，必须先关闭。

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
    {
        public ApplicationDbContext()
            : base("DefaultConnection", throwIfV1Schema: false)
        {
        }

        public DbSet<Announcement> Announcements { get; set; }
        public DbSet<Comment> Comments { get; set; }
        public DbSet<Seen> Seens { get; set; }

        public static ApplicationDbContext Create()
        {
            return new ApplicationDbContext();
        }
    }

Answer 1

事件如果你没有那个错误，你还有其他问题。见下文：

public ActionResult Index() {
string currentUserId = User.Identity.GetUserId();
var currentUser = db.Users.FirstOrDefault( x => x.Id == currentUserId );

List<Seen> seens = new List<Seen>();
if( db.Announcements != null ) {
   foreach( Announcement anoun in db.Announcements ) {
      seens.Add(
      new Seen
      {
         User = currentUser, // You have this already so why go to the database again?
         Announcement = anoun, // Same with this.
      });

   }
}

// Save seens to the database

return View( db.Announcements.ToList() );

Answer 2

该错误是因为您正在积极地迭代从数据库流式传输的集合，然后尝试在该循环内重新向数据库发出另一个选择。它可以通过在使用 ToList() 进入循环之前强制集合的具体化来修复（也可以使用其他选项，例如 AsEnumerable 或 ToArray）。

foreach (Announcement anoun in db.Announcements.ToList())
{
    new Seen
    {
        User = db.Users.Add(currentUser),
        Announcement = db.Announcements.FirstOrDefault(x => x.AnnouncementId == anoun.AnnouncementId),
    };
}

话虽如此，我不确定您为什么要这样做。为什么不直接附加实例anoun，因为您只使用一个（或与显示的代码相同）DbContext 实例（变量名为db）。

foreach (Announcement anoun in db.Announcements)
{
    new Seen
    {
        User = currentUser, // also this seemed wrong before, just assign the reference directly
        Announcement = anoun
    };
}

或者让它更简单：

var newSeens = db.Announcements.Select(x => new Seen(){User = currentUser, Announcement = x}).ToList();
db.Seens.AddRange(newSeens);
db.SaveChanges();

这假设用户没有看到任何公告。如果用户看到了一些，那么您需要根据该用户的现有 Seen 记录过滤 db.Announcements。