named_scope的用法及如何paginate
Nick Kallen颇受欢迎的has_finder插件以named_scope的方式集成到了Rails 2.x版本，例子：
class User < ActiveRecord::Base
named_scope :active, :conditions => {:active => true}
named_scope :inactive, :conditions => {:active => false}
named_scope :recent, lambda { { :conditions => ['created_at > ?', 1.week.ago] } }
end
# Standard usage
User.active # same as User.find(:all, :conditions => {:active => true})
User.inactive # same as User.find(:all, :conditions => {:active => false})
User.recent # same as User.find(:all, :conditions => ['created_at > ?', 1.week.ago])
# They're nest-able too!
User.active.recent
# same as:
# User.with_scope(:conditions => {:active => true}) do
# User.find(:all, :conditions => ['created_at > ?', 1.week.ago])
# end
has_finder中的好处同样可以在named_scope中找到 --- 你还能得到一些额外的好处。 User.all 默认作为 User.find(:all)的别名，可以之间使用。
高级
对某些需求，不要忘了has_finder的某些功能：
传递参数
给你命名的scope传递参数，便于在运行时指定条件。
class User < ActiveRecord::Base
named_scope :registered, lambda { |time_ago| { :conditions => ['created_at > ?', time_ago] }
end
User.registered 7.days.ago # same as User.find(:all, :conditions => ['created_at > ?', 7.days.ago])
Named Scope Extensions
扩展命名的scope（和association_extensions有点类似）。
class User < ActiveRecord::Base
named_scope :inactive, :conditions => {:active => false} do
def activate
each { |i| i.update_attribute(:active, true) }
end
end
end
# Re-activate all inactive users
User.inactive.activate
匿名 scope
你还可以使用默认提供的scoped来构造。（词句不好翻译）
# Store named scopes
active = User.scoped(:conditions => {:active => true})
recent = User.scoped(:conditions => ['created_at > ?', 7.days.ago])
# Which can be combined
recent_active = recent.active
# And operated upon
recent_active.each { |u| ... }
（译者注：这和javascript中的匿名函数相似：var foo = function(x) { alert(x); };）
named_scope是很好的功能。如果你还没有开始用它，现在就开始。你将来会离不开它。还是要感谢一下Nick。
原文地址：http://ryandaigle.com/articles/2008/3/24/what-s-new-in-edge-rails-has-finder-functionality
如何把named_scope和paginate结合起来？
class Product < ActiveRecord::Base
named_scope :online, :conditions => {:status => 1}, :include => [:variants, :catalogue_images, :categories]
named_scope :from_category_ids, lambda { |cat_ids| {:conditions => "categories_products.category_id IN (#{cat_ids})", :include => :categories }}
end
class CatalogueController < RaidBase
def category
@products = Product.from_category_ids(@category.leaf_ids).online.paginate :page => params[:page], :per_page => params[:per_page]
end
end