EntityCommandExecutionException - 試圖鏈接1個數據庫中的2個模型

Question

我有兩個模型 - 一個公告，其中有些人可以發佈公告和一個看到的模型，確定是否有人看到該公告。她EIS型號：

宣佈：

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

，看到：

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

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

在我AnnouncementController.Index

我有這樣的代碼，幾乎應該是，如果您查看此頁，標出每個公告如所見b但在「新看到」部分出現錯誤：

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()); 
} 

已經有一個與此命令關聯的打開的DataReader，它必須先關閉。

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

這個錯誤是因爲你正在積極遍歷集合從數據庫流，然後嘗試重新發出另一個選擇到循環內的數據庫。可以通過在使用ToList()（其他選項也可用，如AsEnumerable或ToArray）進入循環之前強制集合的實現來解決該問題。

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

這就是說我不確定你爲什麼這樣做。爲什麼不直接附加實例anoun，因爲您只使用單個（或顯示的代碼中的相同）DbContext實例（變量名稱爲db）。

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

或使其更簡單：

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

這假定用戶還沒有看到任何公告。如果用戶看到了一些，那麼您需要在該用戶的現有Seen記錄上過濾db.Announcements。

Answer 2

事件如果您沒有該錯誤，您仍然有其他問題。見下：

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());