mongoid scope with referenced object as criteria

I have following scope for Mongoid model in Rails 3:

class Expert
  include Mongoid::Document
  referenced_in :category

   scope :currently_available, lambda { |category, limit|
    limit ||= 5
    {
      :where => {:category => category, :state =开发者_StackOverflow中文版> 'available'}, 
      :limit => limit
    }
  }

category here is an instance of referenced model:

class Category
  include Mongoid::Document
  references_many :experts, :inverse_of => :category

When I call the scope as Expert.currently_available(Category.first, 5), I got a Criteria object:

ruby-1.9.2-p136 :110 > Expert.currently_available(Category.first, 5)
 => #<Mongoid::Criteria
  selector: {:category=>#<Category _id: 4d95ea8773fdea4c47000003, _type: nil, _id: BSON::ObjectId('4d95ea8773fdea4c47000003'), title: "Tax Advisors", price: 5.5>, :state=>"available"},
  options:  {:limit=>5},
  class:    Expert,
  embedded: false>

Question is: How can I load a collection within this criteria? When I do .to_a, it says:

Cannot serialize an object of class Category into BSON

Category itself is valid BSON obj when picked up directly, but in scope it fails to render referencing obj.

Thanks in advance!

This works for me(Mongoid 2.0):

:where => {:category_id => category.id, :state => 'available'}