Question

Best to describe this in code…

I have this

public class A<T>
{
     public static class Queries
     {
          public static Func<DataContext, int, T>Get =
                CompiledQuery.Compile<DataContext, int, T>(
                    (DataContextdb, int i) => (from t in db.GetTable<T>()
                                                      where t.ID == i
                                                      select t).SingleOrDefault());
     }
}

Can I create a new class and query, like this…

public class B<T>:A<t>
{
      public static class BQueries
      {
          public static Func<DataContext, int, T> Get =
              CompiledQuery.Compile<DataContext, int, T>(
                  (DataContext db, int id) => (from t in A.Queries.Get(db, id)
                                               where !t.Item.Deleted 
                                               select t).SingleOrDefault());
      }

      public abstract TrackingItem Item;
}

All I want to do is compile a new query based on the original that adds a couple constraints. This is in lieu of executing two queries

Answer 1

It would work if A.Queries.Get(...) returned an IQueryable<>, but as written your code returns a T. However, if you remove SingleOrDefault() it should work fine.

Update: For good measure, this code should work:

public static class Queries
{
    public static Func<DataContext, int, IQueryable<T>>Get =
        CompiledQuery.Compile<DataContext, int, IQueryable<T>>(
            (DataContext db, int i) => from t in db.GetTable<T>()
                                       where t.ID == i
                                       select t);
}

public static class BQueries
{
    public static Func<DataContext, int, IQueryable<T>> Get =
        CompiledQuery.Compile<DataContext, int, IQueryable<T>>(
            (DataContext db, int id) => from t in A.Queries.Get(db, id)
                                        where !t.Item.Deleted 
                                        select t);
}

The queries won’t actually execute until you call an immediate operator like SingleOrDefault():

using(var db = new MyDataContext())
{
    var res = BQueries.Get(db, id);
    var entity = res.SingleOrDefault();
    // ...
}