MongoPersist.rb

require 'rubygems'
require 'mharris_ext'
require 'mongo'

class OrderedHash
  def reject(&b)
    res = OrderedHash.new
    each { |k,v| res[k] = v unless yield(k,v) }
    res
  end
end

class Object
  def sos(m)
    respond_to?(m) ? send(m) : self
  end
end

class Object
  def safe_to_mongo_hash
    sos(:to_mongo_hash)
  end
  def safe_to_mongo_object
    sos(:to_mongo_object)
  end
end

class Array
  def to_mongo_hash
    map { |x| x.safe_to_mongo_hash }
  end
  def to_mongo_object
    map { |x| x.safe_to_mongo_object }
  end
end


module MongoHash
  def to_mongo_object
    return self unless self['_mongo_class']
    cls = eval(self['_mongo_class'])
    h = reject { |k,v| k == '_mongo_class' || k == '_id' }
    cls.from_mongo_hash(h)
  end
end

[Hash,OrderedHash].each { |cls| cls.send(:include,MongoHash) }



module MongoPersist
  #can be overriden by class.  If not, assumes that all instance variables should be saved.
  def mongo_attributes
    instance_variables.map { |x| x[1..-1] }
  end
  def to_mongo_hash
    mongo_attributes.inject({}) { |h,attr| h.merge(attr => send(attr).safe_to_mongo_hash) }.merge("_mongo_class" => self.class.to_s)
  end
  def from_mongo_hash(h)
    h = h.map_value { |v| v.safe_to_mongo_object }
    from_hash(h)
  end
  def mongo_save!
    self.class.collection.save(to_mongo_hash)
  end
  
  module ClassMethods
    attr_accessor :collection
    def from_mongo_hash(h)
      new.tap { |x| x.from_mongo_hash(h) }
    end
    def mongo_connection(ops)
      ops.each { |k,v| send("#{k}=",v) }
    end
  end
  def self.included(mod)
    super(mod)
    mod.send(:extend,ClassMethods)
  end
end

class ObjCursor
  attr_accessor :cursor
  include FromHash
  def each
    cursor.each { |x| yield(x.to_mongo_object) }
  end
end

class Mongo::Collection
  def find_objects(*args)
    ObjCursor.new(:cursor => find(*args))
  end
end

###### END LIBRARY, START CLIENT CODE THAT USES LIBRARY ###########

def get_coll
  Mongo::Connection.new.db('test-db').collection('orders')
end

class Order
  include FromHash
  include MongoPersist
  mongo_connection :collection => get_coll
  
  attr_accessor :po_number
  fattr(:products) { [] }
end

class Product
  include FromHash
  include MongoPersist
  attr_accessor :sku, :price
end

Order.collection.remove

Order.new(:po_number => 1234, :products => [Product.new(:sku => 'ABC-454', :price => 50)]).mongo_save!
Order.new(:po_number => 1235, :products => [Product.new(:sku => 'ABC-455', :price => 50)]).mongo_save!

Order.collection.find_objects.each { |x| puts x.inspect }
#<Order:0x168e114 @products=[#<Product:0x168dc14 @price=50, @sku="ABC-455">], @po_number=1235>
#<Order:0x168d624 @products=[#<Product:0x168d124 @price=50, @sku="ABC-454">], @po_number=1234>