什麼導致Linq錯誤：這種方法不能被轉換成商店表達式？

Question

我有一堆Linq to Entity方法具有相同的select語句，所以我認爲我會聰明並將它分離出來以減少冗餘的自己的方法...但是當我試圖運行代碼時，我得到了下面的錯誤......

this method cannot be translated into a store expression

這裏是我創建的方法...

public User GetUser(DbUser user, long uid) 
{ 
    return new User 
    { 
     Uid = user.uid, 
     FirstName = user.first_name, 
     LastName = user.last_name 
    }; 
} 

而且我打電話在這樣的方法......

public User GetUser(long uid) 
{ 
    using (var entities = new myEntities()) { 
     return 
      entities.DbUsers.Where(x => x.uid == uid && x.account_status == (short)AccountStatus.Active). 
       Select(x => GetUser(x, uid)).FirstOrDefault(); 
    } 
} 

更新：這裏是工作的直列

public User GetUser(long uid, long uid_user) 
     { 
      using (var entities = new myEntities()) 
      { 

       var q = from u in entities.DbUsers 
         where u.uid == uid_user 
         select new User 
         { 
          Uid = u.uid, 
          FirstName = u.first_name, 
          LastName = u.last_name, 
          BigPicUrl = u.pic_big, 
          Birthday = u.birthday, 
          SmallPicUrl = u.pic_small, 
          SquarePicUrl = u.pic_square, 
          Locale = u.locale.Trim(), 
          IsFavorite = u.FavoriteFriends1.Any(x => x.uid == uid), 
          FavoriteFriendCount = u.FavoriteFriends.Count, 
          LastWishlistUpdate = u.WishListItems.OrderByDescending(x => x.added).FirstOrDefault().added, 
          Sex = (UserSex)u.sex 
         }; 

       var user = q.FirstOrDefault(); 
       user.DaysUntilBirthday = user.Birthday.DaysUntilBirthday(); 
       return user; 
      } 
     } 

Answer 1

你不能這樣做，因爲的getUser方法不能轉換爲任何TSQL語句的代碼。 如果你先回你的DBUSER，然後用它作爲GetUser方法的第一個參數，那麼你正迫使它來執行，一旦你有你DBUser，你可以把它傳遞給GetUser

也許你可以試試這個：

public User GetUser(long uid) 
{ 
    using (var entities = new myEntities()) 
    { 
     return GetUser(
      entities.DbUsers 
       .Where(x => x.uid == uid && x.account_status == (short)AccountStatus.Active) 
       .FirstOrDefault(), 
      uid); 
    } 
} 

編輯

既然你說這仍然失敗難道是怎麼一回事，因爲枚舉的？

public User GetUser(long uid) 
{ 
    using (var entities = new myEntities()) 
    { 
     short status = (short)AccountStatus.Active; 
     return GetUser(
      entities.DbUsers 
       .Where(x => x.uid == uid && x.account_status == status) 
       .FirstOrDefault(), 
      uid); 
    } 
} 

Answer 2

錯誤是現貨，你不能將它翻譯成T-SQL（或P-SQL）查詢。

您需要確保在嘗試將其水合成其他類型之前執行了查詢。

保持簡單，使用擴展方法。這就是他們在那裏。

public static User ToUserEntity(this DbUser user) 
{ 
    return new User 
    { 
     Uid = user.uid, 
     FirstName = user.first_name, 
     LastName = user.last_name 
    }; 
} 

然後在您的DAL：

public User GetUser(long uid) 
{ 
    User dbUser; 

    using (var entities = new myEntities()) 
    { 
     dbUser = entities.DbUsers 
        .Where(x => x.uid == uid && x.account_status == (short)AccountStatus.Active) 
       .FirstOrDefault(); // query executed against DB 
    } 

    return dbUser.ToUserEntity(); 
} 

看我怎麼滋潤POCO爲後上下文已被釋放的對象？通過這種方式，您可以確保EF在嘗試吸入自定義對象之前完成了表達式工作。

另外我不知道你爲什麼傳遞uid到那個方法，它甚至沒有被使用。

進一步說明，你不應該需要來做這種事情（項目EF POCO的到你自己的對象）。

如果你這樣做，這是一個很好的例子定製POCO的（表直接映射到自定義POCO的，不使用代碼生成）。

Answer 3

這個表達式會產生所需的結果（有點）我還沒有想出如何在select語句中傳遞額外的變量......

..... .Select(GetUser).FirstOrDefault()   

static readonly Expression<Func<DbUser, User>> GetUser = (g) => new User { 
      Uid = g.uid, 
      FirstName = g.first_name, 
      LastName = g.last_name, 
      BigPicUrl = g.pic_big, 
      Birthday = g.birthday, 
      SmallPicUrl = g.pic_small, 
      SquarePicUrl = g.pic_square, 
      Locale = g.locale.Trim(), 
      //IsFavorite = g.FavoriteFriends1.Any(x=>x.uid==uid), 
      FavoriteFriendCount = g.FavoriteFriends.Count, 
      LastWishlistUpdate = g.WishListItems.OrderByDescending(x=>x.added).FirstOrDefault().added 
     };