First, I understand that from the point of view of current mongoid errors, I'm doing it wrong. However it's the best solution to the task I have and the problem described here looks easy to fix.

Problem description.
I have three classes.

class USA
  include Mongoid::Document
  embeds_many :states
end

class State
  include Mongoid::Document
  embedded_in :USA
  belongs_to :person
end

class Person
  include Mongoid::Document
  has_many :states
end

So, I can get USA, I can get states in USA. But when I do something along the line of:

USA.states.each do |state|
  state.person
end

I'm getting N queries. Instead I'd like to see the next happening query wise.
Solution:

Person.where( :_id => {:$in => USA.states.map{|state| state.person_id} })

Some mangling will be necessary to map those objects to their related embedded objects, though.

While on the topic I'd like to illustrate that it's possible to use this relation to find all the states that belong to the person, even though Mongoid will throw an error.

class Person
  include Mongoid::Document
  has_many :states
  def self.states
    USA.where('states.person_id' => self._id).map{|usa| usa.states.map{|state| state if state.info_card_id == self._id} }.flatten.compact
  end
end

As it goes, having embedded object referencing other objects may arise and such methods are pretty straightforward to generate on the fly. In my case I need states in my USA class for fast searching, indexing and retrieval while storing references to other classes. And sometimes I need to know states that are related to particular person.