ActiveRecord memo(2)

1. 18.1 p379
<br />#會建立一個可同時在兩個外鍵欄位上搜尋的index, 一般而言便是可以很快找到某個 product_id 所對應的category_id<br />add_index :categories_products, [:product_id, :category_id]<br /><br />#再利用這個index 快速取得此category<br />add_index :categories_products, :category_id<br />

2. 18.3 p384
<br />>> u = User.first<br />=> #<User id: 1, name: "ilakela"><br />>> u.profile.others<br />=> "dddddddddd"<br />>> u.profile.others<br />=> "dddddddddd"<br />>> u.profile.others<br />=> "dddddddddd"<br /><br />#傳回的profile object 結果被cache 了, 所以只撈了一次db<br /> User Load (0.2ms) SELECT * FROM `users` LIMIT 1<br /> Profile Load (0.6ms) SELECT * FROM `profiles` WHERE (`profiles`.user_id = 1) LIMIT 1<br /><br />
3. dependent
<br />#砍上面的, 下面的也砍<br />:dependent => :destroy(or true) <br /><br />#砍上面的, 下面設成foreign key 設成null<br />:dependent => :nullify<br /><br />#砍上面的不理下面的<br />:dependent => :false(or nil)<br />

4 18.3 p389, :finder_sql, and order
<br /> #select A的東西, 卻需要B的條件時<br /> has_many :rails_line_items,<br /> :class_name => "LineItem"<br /> :finder_sql => "select l.* from line_items l, products p" +<br /> "where l.product_id = p.id" +<br /> "and p.title like '%rails%'"<br /><br /> #這樣是先照quantity 的ASC 做排序, 相同時再用unit_price DESC 排序<br /> has_many :line_items, :order => "quantity, unit_price DESC"<br />

5. 18.3 p391 has_many 的一些method, <<, push, replace, clear, uniq(好像拿掉了)

6. 去除重覆的部份
sql distinct
<br /> #unique的動作是在ActiveRecord 做的<br /> has_many :users, :through => :readings, :unique => true<br /><br /> #可以在sql 做好<br /> has_many :users, :through => :readings, :select => "distinct users.*"<br />

7. has_many, extend
<br />#基本<br />#原本<br />class Organization < ActiveRecord::Base<br /> has_many :people do<br /> def find_active<br /> find(:all, :conditions => ["active = ?", true])<br /> end<br /> end<br />end<br /><br />#弄到自己的module<br />module FindActiveExtension<br /> def find_active<br /> find(:all, :conditions => ["active = ?", true])<br /> end<br />end<br /><br />class Organization < ActiveRecord::Base<br /> has_many :people, :extend => FindActiveExtension<br />end<br />#-----------------------------------------------------------<br />#高級技巧 XD<br />#原本的樣子<br />class Project < ActiveRecord::Base<br /> has_many :tasks, :dependent => :delete_all do<br /> def active(reload=false)<br /> @active = nil if reload<br /> @active ||= find(:all, :conditions => "status = 'active'")<br /> end<br /><br /> def inactive(reload=false)<br /> @inactive = nil if reload<br /> @inactive ||= find(:all, :conditions => "status = 'inactive'")<br /> end<br /> end<br />end<br /><br />#利用 extend 的技巧<br />class Project < ActiveRecord::Base<br /> has_many :tasks, :dependent => :delete_all do<br /> memoized_finder :active, "status = 'active'"<br /> memoized_finder :inactive, "status = 'inactive'"<br /> end<br />end<br /><br />#注意他是弄在core module<br />class Module<br /> def memoized_finder(name, conditions=nil)<br /> class_eval <<-STR<br /> def #{name}(reload=false)<br /> @#{name} = nil if reload<br /> @#{name} ||= find(:all, :conditions => #{conditions.inspect})<br /> end<br /> STR<br /> end<br />end<br />