Inheritance in MongoDb: how to request instances of defined type

Question

This is how I used to utilize inheritance in Entity Framework (POCO):

ctx.Animals // base class instances (all instances)
ctx.Animals.OfType  // inher         


        

Answer 1

Well, a document db does in fact store objects "as is" - i.e. without the notion of objects belonging to some particular class. That's why you need _t when you want the deserializer to know which subclass to instantiate.

In your case, I suggest you come up with a discriminator for each subclass, instead of relying on the class name. This way, you can rename classes etc. without worrying about a hardcoded string somewhere.

You could do something like this:

public abstract class SomeBaseClass
{
    public const string FirstSubClass = "first";
    public const string SecondSubClass = "second";
}

[BsonDiscriminator(SomeBaseClass.FirstSubClass)]
public class FirstSubClass { ... }

and then

var entireCollection = db.GetCollection("coll");

var subs = entireCollection.Find(Query.Eq("_t", SomeBaseClass.FirstSubClass));