Using C# MongoDB v2 driver with discriminator and polymorphism

Question

Using C# MongoDB LINQ with discriminator describes exacty what I am trying to do, but I am trying to use the new official 2.0.1 driver.

I have a series of subclasses with their specialised properties all inheriting from a base class containing common properties. Discriminator attributes seem fine and I have successfully saved a mixture of documents using the subtypes.

What I expected to be able to do next was a query similar to:

  var subsetB = db.GetCollection<BaseClass>("Documents").AsQueryable<BaseClass>().OfType<SubclassB>();

To get all the documents of type SubclassB. However, the AsQueryable() and OfType() methods mentioned in the documentation and articles I have found don't seem to be available.

Have I missed something or is there an alternative recommended method of achieving this with the new driver?

Answer 1

There's no special support for that.

You need to explicitly add a filter for the discriminator field, _t.

var results = await collection.Find(Builders<SubclassB>.Filter.Eq("_t", nameof(SubclassB))).ToListAsync();

Answer 2

This seems to work for me in the latest version of the drivers:

var results = await GetCollection<BaseClass>("Documents").OfType<SubClass>.Find(...);