Question

In a database I have Reservations and OldReservations tables, where OldReservations is a copy ( of Reservations table ) and is used to store old reservations. Here are the steps I did to create TPC inheritance:

1) I derived OldReservation entity from Reservations entity
2) I removed the overlapping properties from OldReservations entity
3) I’ve then mapped the OldReservations table fields in the XML of the EDMX file

But when I run the folllowing query, the generated sql uses LEFT OUTER JOIN, which doesn’t make sense, since it means that query will only return rows from OldReservations table where Reservations.ReservationID == OldReservations.ReservationID. From the articles I’ve read it seems that the above query should use an UNION operator and not the LEFT OUTER JOIN:

        var reservations = context.Reservations;
        foreach (var item in reservations);

Generated SQL:

SELECT CASE
         WHEN (NOT (([Project1].[C1] = 1)
                    AND ([Project1].[C1] IS NOT NULL))) THEN '0X'
         ELSE '0X0X'
       END                         AS [C1],
       [Extent1].[ReservationID]   AS [ReservationID],
       [Extent1].[ReservationDate] AS [ReservationDate],
       [Extent1].[ContactID]       AS [ContactID],
       [Extent1].[EventID]         AS [EventID],
       [Extent1].[RowVersion]      AS [RowVersion]
FROM   [dbo].[Reservations] AS [Extent1]
       LEFT OUTER JOIN (SELECT [Extent2].[ReservationID] AS [ReservationID],
                               cast(1 as bit)            AS [C1]
                        FROM   [dbo].[OldReservations] AS [Extent2]) AS [Project1]
         ON [Extent1].[ReservationID] = [Project1].[ReservationID]

TPC select query using left outer join doesn’t make sense, so what am I doing wrong when creating TPC inheritance?

thank you

Answer 1

That’s how TPC is supposed to work. Reservation is a concrete type, so the properties in Reservation are stored in the Reservations table. OldReservation is also a concrete type, so the properties in OldReservation that aren’t already mapped are stored in the OldReservations table.

What you want is similar, but not quite the same. Make Reservation an abstract type and rename it to ReservationBase. Then create a derived type Reservation. Now OldReservation no longer derives from a concrete type, so all of OldReservation own properties and inherited properties will map to the OldReservations table.

After that, yourContext.ReservationBases can be used to get both Reservations and OldReservations, and that should use a UNION (ALL).