How do you scope ActiveRecord associations in Rails 3?

I suggest you take a look at "Named scopes are dead"

The author explains there how powerful Arel is :)

I hope it'll help.

EDIT #1 March 2014

As some comments state, the difference is now a matter of personal taste.

However, I still personally recommend to avoid exposing Arel's scope to an upper layer (being a controller or anything else that access the models directly), and doing so would require:

1. Create a scope, and expose it thru a method in your model. That method would be the one you expose to the controller;
2. If you never expose your models to your controllers (so you have some kind of service layer on top of them), then you're fine. The anti-corruption layer is your service and it can access your model's scope without worrying too much about how scopes are implemented.

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

Instead of scopes I've just been defining class-methods, which has been working great

def self.age0 do
  where("blah")
end