In MVC, pass complex query and view model to session? - mysql

Question

I have a view model:
public class UserCollectionView
{
public CardCollection CardCollections { get; set; }
public Card Cards { get; set; }
}
I have a List View Controller:
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));
I am trying to use the PagedList to add paging to the results. I have been able to do this when I am not using a query that returns data from 2 databases in a single view. As shown here
My end result looks something like this:
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
And so on... I get an error
The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
I have tried variations of SQL statements but can't get it to fit with my view model. In SQL Management Studio this brings back the correct results
Select * from Cards Inner Join CardCollections On Cards.CardID = CardCollections.CardID where CardCollections.UserID = 1 and CardCollections.NumberofCopies > 0;
I need a way to pass the query in session so the paging will operate correctly. Any advice is appreciated. :)

Answer 1

Short answer is, you can't. The model needs to be a snapshot of the content and therefore you can't pass an open query across the boundary (either as a hand-off to a session or to the client directly).
What you're seeing is the disposal of the context beyond it's initial use (where you assemble var viewmodel).
With that said, you can cache the results (to save overhead) if querying the data is an expensive operation. Basically, you'd store the entire collection (or at least a large subset of the collection) in the session/memorycache (which can then be manipulated into a paged list). Something to the effect of:
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

It turns out that I didn't need to pass the query at all. If I let it run through it works fine without the session. I am not really sure why this works but my search query has to be passed. Maybe it is because I am using a viewmodel to perform the query. I will experiment and post if I find anything. Currently the working code is:
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));