class Folder
  include Mongoid::Document
  field :name, :type => String
  has_many :folder_items
end

class FolderItem
  include Mongoid::Document
  belongs_to :folder
  field :name, :type => String
  validates :name, :uniqueness => {:scope => :folder_id} 
end

personal_folder.folder_items << FolderItem.new(:name => "non-unique") 
public_folder.folder_items   << FolderItem.new(:name => "non-unique") 

public_folder.folder_items.first.update_attributes(:folder_id => personal_folder.id)