How to query an Entity Framework entities container for objects of derived entity classes?

Question

I have a complex inheritance structure in my data model. A majority of classes in my model (which is designed in VS 2010 with a DB generated by it after) are derived from 3 basic classes. And only these 3 classes can be found among the context members. How do I use all the derived classes at all?

Answer 1

You can query for subtypes like this:

  var horses = from animal in ctx.AnimalSet
               where animal is Horse
               select animal as Horse;

This will fetch all Horse objects from the Animal set in my examle context.

If you wish to query on sub type specific properties you can do:

var horses = from animal in ctx.AnimalSet
             where animal is Horse //edit, this line is not needed
             let horse = animal as Horse
             where horse.TracksWon > 3
             select horse;

This will all be translated to SQL, so there is no overhead like fetching all animals and filtering on client side, it works as it should.

HTH

Answer 2

Roger's answer kind of works, but might not give you the results you want. It's generally better to use OfType(). To use his examples:

var horses = from animal in ctx.AnimalSet
             where animal is Horse
             select animal as Horse;

This makes horses of type IQueryable<Animal>. But in this case you probably want IQueryable<Horse>, which you can get by:

var horses = from animal in ctx.AnimalSet.OfType<Horse>()
             select animal;

... or just:

var horses = ctx.AnimalSet.OfType<Horse>();

Similarly, Roger's second query can be rewritten as:

var horses = from horse in ctx.AnimalSet.OfType<Horse>()
             where horse.TracksWon > 3
             select horse;

Which is easier to read, but changes the result type to IQueryable<Horse>.