adds a feature to set sequence number to Mongoid::Document

sequence.rb

# encoding: utf-8
module Mongoid
  # Include this module to add automatic sequence feature (also works for _id field, so SQL-Like autoincrement primary key can easily be simulated)
  # usage:
  # class KlassName
  #   include Mongoid::Document
  #   include Mongoid::Sequence
  # ...
  #   field :number, :type=>Integer
  #   sequence :number
  # ...
  module Sequence
    extend ActiveSupport::Concern
        
    module ClassMethods
      def sequence(_field)
        # REPLACE FIELD DEFAULT VALUE
        _field = _field.to_s
        field(_field, fields[_field].options.merge(:default => lambda{ self.class.set_from_sequence(_field)}))
      end
      
      def set_from_sequence(_field)
        sequences = self.db.collection("__sequences")
        counter_id = "#{self.class.name.underscore}_#{_field}"

        # Increase the sequence value and also avoids conflicts
        catch(:value) do
          value = nil
          begin
            value = sequences.find_and_modify(
              :query => {"_id" => counter_id},
              :update=> {"$inc" => {"value" => 1}},
              :new => true,
              :upsert => true
            ).send("[]", "value")
          end while self.first({:conditions => {_field => value}})
          throw :value, value
        end
      end    
      
      def reset_sequence(_field)  
        sequences = self.db.collection("__sequences")
        counter_id = "#{self.class.name.underscore}_#{_field}"
        sequences.find_and_modify(:query => {"_id" => counter_id}, :update=> {"$set" => {"value" => 0}}, :new => true, :upsert => true)
      end
    end    
  end
end

@goncalossilva I'm playing around with this gem again (I had forgotten about it for a month), and it seems to be working but I noticed one issue. The model's find() method only takes an integer, and it errors out if you try and pass it a string.

1.9.2-p290 :022 > Sample.find(1)
=> #<Sample _id: 1, _type: nil, name: nil>

1.9.2-p290 :024 > Sample.find("1")
BSON::InvalidObjectId: illegal ObjectId format: 1

The default Mongoid implementation of find takes only a string. A workaround is to just force the param to be an integer like so:
@sample = Sample.find(params[:id].to_i)

I noticed that in your gem, the id actually becomes an integer whereas in Mongoid, the id is a BSON::ObjectId object. This may be the problem, is there a way to store the sequential numbers (for ids) as BSON::ObjectId instead of as in integer?

1.9.2-p290 :027 > Sample.find(1)._id
=> 1
1.9.2-p290 :028 > Sample.find(1)._id.is_a?(Integer)
=> true

1.9.2-p290 :009 > Event.first._id
=> BSON::ObjectId('4f98e4271d41c827cb000007')
1.9.2-p290 :010 > Event.first._id.is_a?(Integer)
=> false

@paulsfds I believe that's the default behavior when you field :my_id, :type => Integer... the sequencing code doesn't play with any of that. Please open an issue with this if you think it's something else :)