find the gap

document_series.rb

class DocumentSeries < ActiveRecord::Base
 
  #...
   
  #
  # this returns an array of the free counter numbers in the series
  #
  def empty_series_counter_slots
    (self.counter_start .. self.counter_current).to_a - self.documents.collect{|d| d.document_series_counter}
  end

  #...

end