gistfile1.txt

require "mongo_mapper"

MongoMapper.database = 'test'

module FindOrCreate
  def self.included(base)
    base.class_eval do
      extend ClassMethods
    end
  end
    
  module ClassMethods
    def find_or_create(arg)
      first(arg) || create(arg)
    end
  end
end

class Photo
  include MongoMapper::Document

  key :title, String
  key :keywords, Array
end

class Keyword
  include MongoMapper::Document
  include FindOrCreate

  key :keyword, String
  key :photos, Array
end

# and then adding:

Photo.delete_all
Keyword.delete_all

i1 = Photo.create(:title => 'one')
k = Keyword.find_or_create(:keyword => 'trees')
i1.keywords << k.id
k.photos << i1.id
i1.save; k.save

puts Keyword.find(i1.keywords.first).keyword # Find this Photo's first keyword
puts Photo.find(Keyword.first.photos.first).title # Find the first Photo that has the first keyword