named_scopeについて

環境

Rails

3.0.8

確認用モデル生成

author

$ rails g model author name:string 

book

$ rails g model book author:references title:string price:integer

author:referencesで生成するとBookモデルはbelongs_to :author ができます。

class Book < ActiveRecord::Base
  belongs_to :author
end

モデルのnamed_scope

author

class Author < ActiveRecord::Base
  scope :name_is, lambda {|n| where :name => n}
end

book

class Book < ActiveRecord::Base
  belongs_to :author
  scope :cheaper_than,
    lambda {|p| where 'price <= ?', p}
  scope :written_by,
    lambda {|name| joins(:author) & Author.name_is(name)}
end

• &はmerge()のエイリアス

できること

Book.cheaper_than(1000).written_by('david').to_sql
 => "SELECT \"books\".* FROM \"books\" INNER JOIN \"authors\" ON \"authors\".\"id\" = \"books\".\"author_id\" WHERE \"authors\".\"name\" = 'david' AND (price <= 1000)"