gistfile1.rb

# I strongly suggest using object id's instead of auto incrementing id's
# but since you're here, I'm assuming you've already got your heart set!


# First, create a counter via mongodb console
db.counter.insert({_id:"something",count:0})


# Now, automatically use the counter on save
class Something
  include Mongoid::Document
  include Mongoid::Timestamps

  before_save :set_id

  def set_id
    if self.id.class == BSON::ObjectId
      counter = Mongoid.database["counters"].find_and_modify(
        :query => { "_id" => "something" },
        :update => { "$inc" => {"count" => 1} })
      self.id = Integer(counter["count"])
    end
  end

end