klass method of polymorphic association item raises uninitialized constant error

Calling the klass method on a polymorphic association for an instance results in a NameError being raised. This seems to be due to how Mongoid::Association::Relatable#relation_class determines the name. Since it uses the field name when a class is not specified (like in a polymorphic situation) It tries to resolve a class name that doesn't exist. Then tries to call `Module#const_get` on the non-existant class name https://github.com/mongodb/mongoid/blob/f4eb37c9914d83afd27d3ac135afd568e1279a2f/lib/mongoid/association/relatable.rb#L444

The following models should reproduce the error:

class User

  include Mongoid::Document

end

class PolyModel

  include Mongoid::Document

  belongs_to :actor, polymorphic: true

end

model = PolyModel.new(actor: User.new)

model.actor.klass   # raises NameError expecting to return User