In order to look at the next keywords that I wanted to look at (IS OF, OFTYPE, TREAT) I need to introduce some kind of inheritance. I modified my Shippers table in Northwind to try and model inheritance using the table-per-hierarchy model (it's not the only way that you can model inheritance in EF) where you store all the "objects" into a single table and have some discriminating column so that you can tell them apart.

I modified the Shippers table so it now has 2 extra columns on it;

alter table shippers
    add ShipperType int not null default(0)

alter table shippers
    add ExtraInformation nvarchar(50) null

and set the data so it looks like this;

1    Speedy Express    (503) 555-9831    0    NULL
2    United Package    (503) 555-3199    0    NULL
3    Federal Shipping    (503) 555-9931    1    Some Extra Information

And used egmgen.exe to refresh my northwind.csdl, northind.ssdl, northwind.msl files. I then need to edit them to introduce the inheritance relationship.

So, we have rows 1 and 2 which are regular old Shippers and we've got row 3 which is what I'll be calling a FancyShipper. Now, I then went off and spent about 30 minutes trying to hack my mapping files and I kept coming up with an error that read something but not exactly like this;

ERROR (3020): Problem in Mapping Fragment(s) starting at line(s) {81, 23}: Entity data will be lost when storing in table Shippers: The rows in table Shippers for when [Shippers.ShipperType = 0] and the rows in this table for when [Shippers.ShipperType = 0] are disjoint but the corresponding entities in set(s) (Shippers, FancyShippers) are not necessarily disjoint.

Now, I hadn't the foggiest clue what this was all about but in the end I figured out what it was I was trying to do. In my EDM I'd constructed two EntitySets named Shippers and FancyShippers.

NO, NO, NO!

This is wrong, what I want is one EntitySet called Shippers and that EntitySet contains both Shippers and FancyShippers. If you've got derived types like this and you put them into multiple EntitySets then I think you'll see the error above.

So, what did I actually change to make this work.

northwind.CSDL file

Simply added a new entity type;

  <EntityType Name="FancyShippers" BaseType="northwind.Shippers">
    <Property Name="ExtraInformation" Type="String" Nullable="true" MaxLength="50"/>
  </EntityType>

northwind.SSDL file

No changes necessary, this is all conceptual stuff that the store doesn't care about.

northwind.MSL file

Changed the EntitySetMapping for Shippers to look like this;

    <EntitySetMapping Name="Shippers" StoreEntitySet="Shippers">
      <EntityTypeMapping TypeName="northwind.Shippers">
        <MappingFragment StoreEntitySet="Shippers">
            <ScalarProperty Name="ShipperID" ColumnName="ShipperID" />
            <ScalarProperty Name="CompanyName" ColumnName="CompanyName" />
            <ScalarProperty Name="Phone" ColumnName="Phone" />
            <Condition ColumnName="ShipperType" Value="0"/>
        </MappingFragment>
      </EntityTypeMapping>
      <EntityTypeMapping TypeName="northwind.FancyShippers">
        <MappingFragment StoreEntitySet="Shippers">
          <ScalarProperty Name="ShipperID" ColumnName="ShipperID" />
          <ScalarProperty Name="CompanyName" ColumnName="CompanyName" />
          <ScalarProperty Name="Phone" ColumnName="Phone" />
          <ScalarProperty Name="ExtraInformation" ColumnName="ExtraInformation" />
          <Condition ColumnName="ShipperType" Value="1"/>
        </MappingFragment>
      </EntityTypeMapping>
    </EntitySetMapping>

and with that in place I made sure that my new files validated using the (edmgen.exe /mode:validateartifacts /incsdl:northwind.csdl /inssdl:northwind.ssdl /inmsl:northwind.msl) command line and everything is fine and I'm ready to go with some limited inheritance stuff.