Question

I’m using Entity Framework 4.1’s code first approach to map a class hierarchy to a series of tables.

I have the following classes defined:

[Table("FooBases")]
public class FooBase
{
    public int Id { get; set; }
    public string Name { get; set; }
}

[Table("Foo1s")]
public class Foo1 : FooBase
{
    public string Details { get; set; }
}

[Table("Foo2s")]
public class Foo2 : FooBase
{
    public string Description { get; set; }
}

So this will create three tables, where the common properties (Id and Name) are stored in a FooBase table.

In one particular case I am only interested in a list of names of any kind of FooBase object. I don’t care whether it’s a Foo1 or a Foo2. I spin up a new data context and run a query like this:

var names = ctx.Set<FooBase>().Select(f => f.Name);

Now, I don’t need the overhead of querying the Foo1s and Foo2s tables. I expect the query to just be a simple SELECT Name FROM FooBases. Instead, I get this:

SELECT 
[Extent1].[Name] AS [Name]
FROM  [dbo].[FooBases] AS [Extent1]
LEFT OUTER JOIN  (SELECT 
    [Extent2].[Id] AS [Id]
    FROM [dbo].[Foo1s] AS [Extent2]
UNION ALL
    SELECT 
    [Extent3].[Id] AS [Id]
    FROM [dbo].[Foo2s] AS [Extent3]) AS [UnionAll1] ON [Extent1].[Id] = [UnionAll1].[Id]

Is there a way to tell Entity Framework that I don’t care about the other tables; that I just want data from the top level of the class hierarchy so there’s no need to query anything other than the top level table?

Answer 1

This is well known issue with Table-per-type inheritance mapping. It should be addressed in upcoming EFv4.2 which should improve the query generation for TPT inheritance. At the moment there is no way to improve it.