在 MVC 中，将复杂的查询和视图模型传递给会话？

Question

我有一个视图模型：

 public class UserCollectionView
    {
        public CardCollection CardCollections { get; set; }
        public Card Cards { get; set; }    
    }

我有一个列表视图控制器：

public ActionResult ViewCollection(int? page)
        {
            var userid = (int)WebSecurity.CurrentUserId;
            var pageNumber = page ?? 1;
            int pageSize = 5;                
            ViewBag.OnePageOfCards = pageNumber;
            if (Session["CardCollection"] != null)
            {    
             var paging = Session["CardCollection"].ToString.();
             return View(paging.ToPagedList(pageNumber, pageSize));    
            }                                  
            var viewModel = from c in db.Cards
                            join j in db.CardCollections on c.CardID equals j.CardID
                            where (j.NumberofCopies > 0) && (j.UserID == userid)
                            orderby c.Title
                            select new UserCollectionView { Cards = c, CardCollections = j };

            Session["CardCollection"] = viewModel;
            return View(viewModel.ToPagedList(pageNumber, pageSize));

我正在尝试使用 PagedList 将分页添加到结果中。当我不使用从单个视图中返回来自 2 个数据库的数据的查询时，我已经能够做到这一点。 As shown here

我的最终结果如下所示：

     Cards.SeveralColumns   CardCollections.ColumnA   CardCollections.ColumnB 
Row 1 Data from Cards Table  A from CardCollections    B from CardCollections
Row 2 Data from Cards Table  A from CardCollections    B from CardCollections
Row 3 Data from Cards Table  A from CardCollections    B from CardCollections

等等...我收到一个错误 The ObjectContext instance has been disposed and can no longer be used for operations that require a connection. 我已经尝试过各种 SQL 语句，但无法使其适合我的视图模型。在 SQL Management Studio 中，这会返回正确的结果 Select * from Cards Inner Join CardCollections On Cards.CardID = CardCollections.CardID where CardCollections.UserID = 1 and CardCollections.NumberofCopies > 0;

我需要一种在会话中传递查询的方法，以便分页正常运行。任何建议表示赞赏。 :)

Answer 1

简短的回答是，你不能。模型需要是内容的快照，因此您不能跨边界传递打开的查询（作为会话的移交或直接传递给客户端）。

您所看到的是上下文在其初始使用之外的处置（您在其中组装var viewmodel）。

话虽如此，如果查询数据是一项昂贵的操作，您可以缓存结果（以节省开销）。基本上，您会将整个集合（或至少集合的一个大子集）存储在会话/内存缓存中（然后可以将其操作到分页列表中）。大意是：

public ActionResult ViewCollection(int? page)
{
  var userId = (int) WebSecurity.CurrentUserId;
  var pageNumber = page ?? 1;
  var pageSize = 5;

  ViewBag.OnePageOfCards = pageNumber;

  var cacheKey = String.Format("ViewCollection[{0}]", userId);
  var entities = GetOrCreateCacheItem<IEnumerable<UserCollectionView>>(cacheKey, () => {
    var query = (from c in db.Cards
                 join j in db.CardCollections on c.CardID equals j.CardID
                 where (j.NumberofCopies > 0) && (j.UserID == userid)
                 orderby c.Title
                 select new UserCollectionView { Cards = c, CardCollections = j }
                )
                .ToList(); // Force fetch from Db so we can cache
  });

  return View(entities.ToPagedList(pageNumber, pageSize));
}

// To give an example of a cache provider; Feel free to change this,
// abstract it out, etc.
private T GetOrCreateCacheItem<T>(string cacheKey, Func<T> getItem)
{
  T cacheItem = MemoryCache.Default.Get(cacheKey) as T;
  if (cacheItem == null)
  {
    cacheItem = getItem();
    var cacheExpiry = DateTime.Now.AddMinutes(5);
    MemoryCache.Default.Add(cacheKey, cacheItem, cacheExpiry);
  }
  return cacheItem;
}

Answer 2

事实证明，我根本不需要传递查询。如果我让它运行，它可以在没有会话的情况下正常工作。我不太确定为什么会这样，但我的搜索查询必须通过。也许是因为我使用视图模型来执行查询。如果我发现任何东西，我会进行实验并发布。目前的工作代码是：

public ActionResult ViewCollection(int? page)
        {
            var userid = (int)WebSecurity.CurrentUserId;
            var pageNumber = page ?? 1;
            int pageSize = 5;

            ViewBag.OnePageOfCards = pageNumber;
            ViewBag.Rarity_ID = new SelectList(db.Rarities, "RarityID", "Title");
            ViewBag.MainType_ID = new SelectList(db.MainTypes, "MainTypeID", "Title");
            ViewBag.CardSet_ID = new SelectList(db.CardSets, "CardSetID", "Title");
            ViewBag.SubType_ID = new SelectList(db.SubTypes, "SubTypeID", "Title");
            var viewModel = from c in db.Cards
                            join j in db.CardCollections on c.CardID equals j.CardID
                            where (j.NumberofCopies > 0) && (j.UserID == userid)
                            orderby c.Title
                            select new UserCollectionView { Cards = c, CardCollections = j };
            return View(viewModel.ToPagedList(pageNumber, pageSize));