"Unable to create a constant value of type .. Only primitive types are supported ..'' in EF query? - entity-framework

Question

I have a work around for this issue, however I would appreciate it if someone could explain why this is happening and how I would design this for large datasets where my work around would not be viable.
The full error is:
Unable to create a constant value of type 'THPT_Razor.Models.WinType'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.
and I am using EF v4.0.
The commented lines are the offending code and the work around is the "For loop"
Thank you in advance.
List<WinType> _atype = db.WinTypes.Where(wt => wt.IsWin == false).ToList();
List<WinType> _wtype = db.WinTypes.Where(wt => wt.IsWin == true).ToList();
string test = _wtype.Where(wt => wt.Value ==0).Select(wt => wt.Description).SingleOrDefault();
List<WinCheckDetails> wcd = db.Wins.Include("UserProfiles").Where(w => w.venueLogId == logid).Select(w => new WinCheckDetails
{
//awarddesc = w.atypeid.HasValue ? _atype.Where( wt=> wt.Value == w.atypeid).Select(wt => wt.Description).SingleOrDefault():string.Empty,
//windesc = _wtype.Where(wt => wt.Value == w.typeid).Select(wt => wt.Description).Single(),
atypeid = w.atypeid,
typeid = w.typeid,
WinId = w.WinId,
other = w.other,
posterid = w.posterid,
confirmed = w.confirmed,
posttime = w.posttime,
game = w.game,
playerid = w.UserProfile.PlayerID,
firstname = w.UserProfile.FirstName,
lastname = w.UserProfile.LastName,
fullname = w.UserProfile.FirstName + " " + w.UserProfile.LastName
}).OrderBy(o => o.game).ToList();
foreach (WinCheckDetails wc in wcd)
{
wc.awarddesc = _atype.Where(wt => wt.Value == wc.atypeid).Select(wt => wt.Description).SingleOrDefault();
wc.windesc = _wtype.Where(wt => wt.Value == wc.typeid).Select(wt => wt.Description).SingleOrDefault();
}

Answer 1

_atype and _wtype are lists of WinType in memory because you are applying ToList() to the queries. With respect to database queries they are collections of constant values because to perform the query in the database they have to be transmitted to the database server as the values they are in memory. EF doesn't support to transfer such constant values or collections of values from memory to the database unless they are values of primitive types (int for example). That's the reason why you get an exception.
Did you try to use _atype and _wtype as IQueryable instead of lists:
IQueryable<WinType> _atype = db.WinTypes.Where(wt => !wt.IsWin);
IQueryable<WinType> _wtype = db.WinTypes.Where(wt => wt.IsWin);
List<WinCheckDetails> wcd = db.Wins
.Where(w => w.venueLogId == logid)
.Select(w => new WinCheckDetails
{
awarddesc = w.atypeid.HasValue
? _atype.Where(wt=> wt.Value == w.atypeid)
.Select(wt => wt.Description).FirstOrDefault()
: string.Empty,
windesc = _wtype.Where(wt => wt.Value == w.typeid)
.Select(wt => wt.Description).FirstOrDefault(),
// ... (unchanged)
}).OrderBy(o => o.game).ToList();
I have removed the Include because it will be ignored anyway when you perform a projection with Select. Also I have replaced SingleOrDefault and Single by FirstOrDefault because both are not supported in a projection (and First neither), only FirstOrDefault is supported.
I am not sure if that will work. But it should remove your exception (but maybe you'll get another one...).