LINQ-to-entities generic == workaround

Question

I have a following LINQ-to-entities query

IQueryable<History<T>> GetFirstOperationsForEveryId<T>
    (IQueryable<History<T>> ItemHistory)
{
    var q = (from h in ItemHistory
             where h.OperationId ==
                (from h1 in ItemHistory
                 where h1.GenericId == h.GenericId
                 select h1.OperationId).Min()
             select h);
    return q;
}

ItemHistory is a generic query. It can be obtained in the following way

var history1 = MyEntitiySet1.Select(obj =>
    new History<long>{ obj.OperationId, GenericId = obj.LongId });
var history2 = AnotherEntitiySet.Select(obj =>
    new History<string>{ obj.OperationId, GenericId = obj.StringId });

In the end of all I want a generic query being able to work with any entity collection convertible to History<T> .

The problem is the code does not compile because of GenericId comparison in the inner query (Operator '==' cannot be applied to operands of type 'T' and 'T').

If I change == to h1.GenericId.Equals(h.GenericId) I get the following NotSupportedException :

Unable to cast the type 'System.Int64' to type 'System.Object'. LINQ to Entities only supports casting Entity Data Model primitive types.

I've tried to do grouping instead of subquery and join the results.

IQueryable<History<T>> GetFirstOperationsForEveryId<T>
    (IQueryable<History<T>> ItemHistory)
{
    var grouped = (from h1 in ItemHistory
                   group h1 by h1.GenericId into tt
                   select new
                   {
                        GenericId = tt.Key,
                        OperationId = tt.Min(ttt => ttt.OperationId)
                   });

    var q = (from h in ItemHistory
             join g in grouped
                on new { h.OperationId, h.GenericId }
                equals new { g.OperationId, g.GenericId }
             select h);
    return q;
}

It compiles because GenericId's are compared with equals keyword and it works but the query with real data is too slow (it has been running for 11 hours on dedicated postgresql server).

There is an option to build a whole expression for the outer where statement. But the code would be too long and unclear.

Are there any simple workarounds for equality comparison with generics in LINQ-to-entities?

Answer 1

Try this, I think it should accomplish what you want without the extra query/join

IQueryable<History<T>> GetFirstOperationsForEveryId<T>
    (IQueryable<History<T>> ItemHistory)
{
  var q = from h in ItemHistory
          group h by h.GenericId into tt
          let first = (from t in tt
                        orderby t.GenericId
                        select t).FirstOrDefault()
          select first;

  return q;
}

Answer 2

IQueryable<History<T>> GetFirstOperationsForEveryId<T>
(IQueryable<History<T>> ItemHistory)
{
var grouped = (from h1 in ItemHistory
               group t by h1.GenericId into tt
               select new
               {
                    GenericId = tt.Key,
                    OperationId = tt.Min(ttt => ttt.OperationId)
               });

var q = (from h in ItemHistory
         join g in grouped
            on new { h.OperationId, h.GenericId }
            equals new { g.OperationId, g.GenericId }
         select h);
return q;
}

Answer 3

您还可以在I上为实现GenericId和OperationId属性的IItemHistory接口设置通用约束。

Answer 4

My question already contains a solution. The second method with group + join works well if the table is properly indexed . It takes 3.28 seconds to retrieve 370k rows from the database table. In fact in non-generic variant the first query is slower on postgresql than the second one. 26.68 seconds vs 4.75.