Mat Harvard

One of (many) cool features in Rails is the named_scope method (in Rails 3.0, this is called scope!).

Article.find(
  :all,
  :conditions => ["published_at IS NOT ?", nil],
  :limit => 5,
  :order => "published_at DESC"
)

Here's a perfectly ordinary and valid use of the find method to query all records in the database with values in their published_at column with a maximum limit of 5 records.

With a named_scope, this could be:

Article.recently_published

In the model:

class Article < ActiveRecord::Base
  named_scope :recently_published,
              :conditions => ["published_at IS NOT ?", nil],
              :limit => 5,
              :order => "published_at DESC"
end

It keeps controllers looking clean and lean. In a completely hypothetical situation, a problem could arise when for instance a varying number of records need to be pulled from the database. Instead of 5, maybe 10, or 11! named_scope's could be written for all of them, but a more simple solution is to just make a utility scope.

named_scope :limit, lambda { |limit| { :limit => limit } }

With this it's possible to do things like: Article.recently_published.limit(10), Article.find(:all).limit(9) or even Article.unpublished.limit(10000). There are multitudes of ways named_scope can be used. Check out Railscasts episode #108 for more tricks.