How do you scope ActiveRecord associations in Rails 3?

Question

I have a Rails 3 project. With Rails 3 came Arel and the ability to reuse one scope to build another. I am wondering if there is a way to use scopes when defining a relationsh

Answer 1

How about association extensions?

class Item < ActiveRecord::Base
  has_many :orders do
    def for_user(user_id)
      where(user_id: user_id)
    end
  end
end

Item.first.orders.for_user(current_user)

UPDATE: I'd like to point out the advantage to association extensions as opposed to class methods or scopes is that you have access to the internals of the association proxy:

proxy_association.owner returns the object that the association is a part of. proxy_association.reflection returns the reflection object that describes the association. proxy_association.target returns the associated object for belongs_to or has_one, or the collection of associated objects for has_many or has_and_belongs_to_many.

More details here: http://guides.rubyonrails.org/association_basics.html#association-extensions