Mongoid presence_of embedded document and destroy embedded

Question

I have some mongoid document:

class Firm
  include Mongoid::Document

  embeds_many :offices
  validates_presence_of :offices
end

At least one office must be present. It works. However when 'destroy' method called for latest office than firm was saved but not valid anymore..

I can use something like this:

class Office
    embedded_in :firm
    before_destroy :check_for_latest

    def check_for_latest
       false if firm.offices.count == 1
    end
end

but it's not good way

Any ideas? Thanks!