Activerecord memo (1)

1. 如果table name不合 rails 的規則時, 可以自己設定<br />set_table_name "user"<br />#或<br />self.table_name = "user"<br />
2. ActirveRecord 提供了一些跟欄位相關的method<br />#列出所有欄位<br />>> User.column_names<br />=> ["id", "name", "created_at", "updated_at"] <br /><br />#列出欄位的詳細資訊<br />>> User.columns_hash["name"]<br />=> #<ActiveRecord::ConnectionAdapters::MysqlColumn:0xb70f4274 @type=:string, @null=true, @default=nil, @name="name", @scale=nil, @sql_type="varchar(255)", @precision=nil, @primary=false, @limit=255><br /><br />#取到全部的Column Object<br />>> User.columns<br />=> [#<ActiveRecord::ConnectionAdapters::MysqlColumn:0xb70f4468 @type=:integer, @null=false, @default=nil, @name="id", @scale=nil, @sql_type="int(11)", @precision=nil, @primary=true, @limit=4>, #<ActiveRecord::ConnectionAdapters::MysqlColumn:0xb70f4274 @type=:string, @null=true, @default=nil, @name="name", @scale=nil, @sql_type="varchar(255)", @precision=nil, @primary=false, @limit=255>, #<ActiveRecord::ConnectionAdapters::MysqlColumn:0xb70f3f90 @type=:datetime, @null=true, @default=nil, @name="created_at", @scale=nil, @sql_type="datetime", @precision=nil, @primary=false, @limit=nil>, #<ActiveRecord::ConnectionAdapters::MysqlColumn:0xb70f3dec @type=:datetime, @null=true, @default=nil, @name="updated_at", @scale=nil, @sql_type="datetime", @precision=nil, @primary=false, @limit=nil>]<br />
3. 除了table column的屬性之外, model 也可以拿到其他的屬性
<br />#利用attributes 可以拿到該object全部的屬性<br /> User.first.attributes<br />=> {"name"=>"AA", "updated_at"=>Thu Jan 29 03:28:01 UTC 2009, "id"=>1, "created_at"=>Thu Jan 29 03:28:01 UTC 2009}<br />>> User.find(:first, :select => 'name').attributes<br />=> {"name"=>"AA"}<br /><br />#透過計算得到的欄位類型是在mysql 是字串, 當然會根據database 的不同而有所不同<br />>> Cost.find_by_sql("select pay_amount, total_amount - consume_amount from costs limit 1")[0].attributes<br />=> {"pay_amount"=>10030.0, "total_amount - consume_amount"=>"10030"}<br />#"total_amount - consume_amount" 這樣的attribute name 太醜 as 一下<br />>> Cost.find_by_sql("select pay_amount, total_amount - consume_amount as sum_amount from costs limit 1")[0].attributes<br />=> {"cal_total"=>10030.0, "pay_amount"=>10030.0}<br /><br />#由於計算的結果得到的欄位在屬性hash 是用String 去存的, 所以當你需要將他用來運算時, 請overwrite 它的reader<br /><br />>> c = Cost.find_by_sql("select pay_amount, total_amount - consume_amount as sum_amount from costs")<br />=> [#<Cost pay_amount: 10030.0>, #<Cost pay_amount: 0.0>, #<Cost pay_amount: 0.0>]<br /><br />#結果是String<br />>> c[0].sum_amount<br />=> "10030"<br /><br />#所以或許妳會想overwrite它, 這時就需要用到read_arrtibute的屬性<br /> def sum_amount <br /> Float(read_attribute('sum_amount'))<br /> end<br /><br />#結果便是float 了<br />>> c[0].sum_amount<br />=> 10030.0<br />
4. Facade column, 有些時候欄位的格式不是最好用的, 如上面的浮點數 string, 這時就可以利用read_attribute 和 write_attribute來達到
<br />#這是個把腕尺(cubit)換成英寸的例子<br />class ProductData < ActiveRecord::Base<br /> CUBITS_TO_INCHES = 18<br /> def length<br /> read_attribute("length" ) * CUBITS_TO_INCHES<br /> #用length * * CUBITS_TO_INCHES 這樣會不行的, 自己call 自己叫不完<br /> end<br /><br /> def length=(inches)<br /> write_attribute("length" , Float(inches) / CUBITS_TO_INCHES)<br /> end<br />end<br /><br />#You can alternatively use self[:attribute]=(value) and self[:attribute] instead of write_attribute(:attribute, value) and read_attribute(:attribute) as a shorter form.<br />
5. before_type_cast, ActiveRecord 會把 attribute reader (無論是attr_reader 或 read_attribute)讀回來的東西強制轉成Ruby type(根據database 的column type 去決定想轉成啥), <br />#像這樣利用before_type_cast, 我們才有辦法得到它在資料庫未轉成Ruby object的值<br />>> User.first.created_at<br />=> Thu, 29 Jan 2009 03:28:01 UTC +00:00<br />>> User.first.created_at_before_type_cast<br />=> "2009-01-29 03:28:01"<br />
6. primary_key 也可以自己指定, 而我們仍是用 id 這樣的屬性去呼叫這個primary key
<br /> self.primary_key = "isbn"<br />
7. find object 的時候如果是直接照主鍵去找的才會丟 RecordNotFound
<br />>> User.find(5)<br />ActiveRecord::RecordNotFound: Couldn't find User with ID=5<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:1528:in `find_one'<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:1511:in `find_from_ids'<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:590:in `find'<br /> from (irb):73<br />>> User.find_by_name("lala")<br />=> nil<br />#多個! 也可以讓它丟<br />>> User.find_by_name!("lala")<br />ActiveRecord::RecordNotFound: ActiveRecord::RecordNotFound<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:1814:in `find_by_name!'<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:1800:in `send'<br /> from /usr/lib/ruby/gems/1.8/gems/activerecord-2.2.2/lib/active_record/base.rb:1800:in `method_missing_without_paginate'<br /> from /home/lake/rails_app/haurti/vendor/plugins/will_paginate/lib/will_paginate/finder.rb:167:in `method_missing'<br /> from (irb):75<br />>> User.find(:first, :conditions => {:name =>"lala"})<br />=> nil<br />>> User.find(:first, :conditions => {:id =>"lala"})<br />=> nil<br />>> User.find(:first, :conditions => {:id => 5})<br />=> nil<br /><br />看一下log<br /> User Load (0.5ms) SELECT * FROM `users` WHERE (`users`.`id` = 5) <br /> User Load (1.6ms) SELECT * FROM `users` WHERE (`users`.`name` = 'lala') LIMIT 1<br /> User Load (0.9ms) SELECT * FROM `users` WHERE (`users`.`id` = 'lala') LIMIT 1<br /> User Load (0.5ms) SELECT * FROM `users` WHERE (`users`.`id` = 5) LIMIT 1<br /><br />所以就是差在 limit 阿<br />