why doesn't .where() honor polymorphism?

I love the query interface (ActiveMethod::QueryMethods), but one thing
has bugged me: why doesn't it generate calls for polymorphic
associations? For example:

class MyModel < ActiveRecord::Base
  belongs_to :parent, :polymorphic => true
end

I would expect:

  MyModel.where(:parent => a)

to be equivalent to:

  MyModel.where("my_models.parent_id = ? AND my_models.parent_type = ?",
                           a.id, a.class.base_class.name)

Is there a philosophical reason it doesn't make this transformation?

- ff

P.S.: After writing this, I realize it's not just for polymorphism. I
would expect the NON-polymorphic case to transform:

  MyModel.where(:parent => a)

into

  MyModel.where("my_models.parent_id = ?", a.id)

When you write “belongs_to :parent” Rails expects there to be a class called Parent.

If you want this to be related to the MyModel class, you’ll need to explicitly state that:

belongs_to :parent, :class_name => ‘MyModel’