什么导致Linq错误：这种方法不能被转换成商店表达式？

Question

我有一堆Linq to Entity方法具有相同的select语句，所以我认为我会聪明并将它分离出来以减少冗余的自己的方法...但是当我试图运行代码时，我得到了下面的错误......

this method cannot be translated into a store expression

这里是我创建的方法...

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

而且我打电话在这样的方法......

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：这里是工作的直列

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

你不能这样做，因为的getUser方法不能转换为任何TSQL语句的代码。 如果你先回你的DBUSER，然后用它作为GetUser方法的第一个参数，那么你正迫使它来执行，一旦你有你DBUser，你可以把它传递给GetUser

也许你可以试试这个：

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); 
    } 
} 

编辑

既然你说这仍然失败难道是怎么一回事，因为枚举的？

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

错误是现货，你不能将它翻译成T-SQL（或P-SQL）查询。

您需要确保在尝试将其水合成其他类型之前执行了查询。

保持简单，使用扩展方法。这就是他们在那里。

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

然后在您的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(); 
} 

看我怎么滋润POCO为后上下文已被释放的对象？通过这种方式，您可以确保EF在尝试吸入自定义对象之前完成了表达式工作。

另外我不知道你为什么传递uid到那个方法，它甚至没有被使用。

进一步说明，你不应该需要来做这种事情（项目EF POCO的到你自己的对象）。

如果你这样做，这是一个很好的例子定制POCO的（表直接映射到自定义POCO的，不使用代码生成）。

Answer 3

这个表达式会产生所需的结果（有点）我还没有想出如何在select语句中传递额外的变量......

..... .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 
     };