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

class FolderItem
  include Mongoid::Document
  include Mongoid::Paranoia
  belongs_to :folder
  field :name, :type => String
  validates :name, :uniqueness => {:scope => :folder_id, :message => 'buyaaa', :case_sensitive => false} 
end

fun = Folder.create(:name => 'Entertainment')
work = Folder.create(:name => 'Work')
music = FolderItem.create(:name => 'Music', :folder_id => fun.id)
movie = FolderItem.new(:name => 'Movies', :folder_id => fun.id)
movie.valid?
movie.save
movie.delete
movie = FolderItem.new(:name => 'Movies', :folder_id => fun.id)

movie.valid?
# MONGODB (0ms) cloudfactory_development['$cmd'].find({"count"=>"folder_items", "query"=>{:name=>"Movies", :folder_id=>BSON::ObjectId('4feafbaa6f90cb295200001e')}, "fields"=>nil}).limit(-1)
=> false