Reading through Rails Recipes: Rails 3 Edition I discovered a much better way of doing my searches.

In my multi-domain application, articles belong to sites, sites have many articles. I was finding articles that belonged to particular sites by doing

Articles.where("site_id = ?", site.id)

It's not too bad, but it gets a bit much writing out a lot of times.

Now, I've added a scope. In my article model I have

app/models/article.rb
class Article < ActiveRecord::Base
  belongs_to :site
  scope :by_site, lambda {|site| where(:site_id => site.id) }
end

Then you can write

Articles.by_site(site)

and get the same thing as before. Much clearer and also DRYer.