Ownable gem helps you verify the "ownership" of a associated models in Rails 3.

The term "ownership" in the contact of Ownable is the association between two models, usually a User Model (though any model can do) and some other Model.

Ownable exposes an instance method on ActiveRecord objects that declare themselves to be acts_as_ownable:

owned_by? (and its alias belongs_to?)

For example, assume the associations:

User has_many Orders has_one Invoice has_many Payments

or in the opposite direction:

Payments belongs_to Invoice belongs_to Orders belongs_to User

Determine if a User "owns" a particular Payment object

Rather than manually defining and then checking the association between User and Payment like:
	@payment.invoice.order.user == current_user # => true/false
	
you can use Ownable for a much shorter and simpler syntax:
   @payment.owned_by? current_user # => true/false

This gem is most helpful when many association steps exist between Models, such as:

A belongs_to B belongs_to ... belongs_to Y belongs_to Z

And the ownership check can be written short hand via Ownable as:

@a.owned_by? @z # => true/false

or

@a.belongs_to? @z # => true/false

Ownable is currently available only in Gem form and requires version >= 3.0 of Rails.

You just have to add the 'ownable' gem to your Gemfile

gem 'ownable'

Then tell bundler to update the gems :

$ bundle install

In each Model define the associations from the Model to the "Owner" model (which will be User in this example)

Assuming a set of Model associations as follows:

User has_many Orders has_one Invoice has_many Payments

Ownable is added to the Payment Model using "acts_as_ownable"

class Payment < ActiveRecord::Base  
  acts_as_ownable :through => [:invoice, :order, :user]
	...
end

The "acts_as_ownable" method call takes a hash with key :through with an Array value. The Array is a list of the Model names that define the association path from the current Model back to, and including the "Owner" model. Important note: The last item in the array is always the "Owner" model name.

Now all Payment objects will have the instance methods:

@payment.owned_by?(ActiveRecord Object)
@payment.owned_by?(Fixnum Id)

or using the belongs_to? alias

@payment.belongs_to?(ActiveRecord Object)
@payment.belongs_to?(Fixnum Id)

Note that when declaring acts_as_ownable in the Models, the last Model name in the array must be the "Owner" model name. Also, this only works when each upstream association returns a single object. For example, the following will NOT work:

Publisher has_many Authors has_and_belongs_to_many Books 


class Book < ActiveRecord::Base  
  acts_as_ownable :through => [:author, :publisher]
	...
end


@book.owned_by? Publisher.first

This will not work because several authors may have co-authored a Book so there is multiple paths back to the "Owner" model (the Publisher). At this time this gem does not support these relationships.

Thanks to jlecour's (http://github.com/jlecour) geokit-rails project for helping me understand how to build a gem that plays well with Rails 3.