Rails sql延迟加载和自带缓存

color_lot_manuallies = color_lot.color_lot_manuallies
if color_lot_manuallies.size == 1
end

引用

SELECT count(*) AS count_all FROM `color_lot_manuallies` WHERE (`color_lot_manuallies`.color_lot_id = 237)

当有A has many B时，a.bs.size并不是检索出所有a.bs再求出size，而是直接用select count(*)来计算结果。这应该是延迟加载，有点类似named_scope,但是并不是named_scope,named_scope的结果类是ActiveRecord::NamedScope::Scope，但是A.bs.class的结果是Array，这部分看了下源码但是并没有看懂。
所以当我这样写的时候：

    color_lot_manuallies = color_lot.color_lot_manuallies
    if color_lot_manuallies.size == 1
    end
    color_lot.color_lot_manuallies.each do |i|
       puts i.id
    end

引用

SQL (0.6ms)   SELECT count(*) AS count_all FROM `color_lot_manuallies` WHERE (`color_lot_manuallies`.color_lot_id = 200)
ColorLotManually Load (0.1ms)   SELECT * FROM `color_lot_manuallies` WHERE (`color_lot_manuallies`.color_lot_id = 200)

这样的话rails使用了延迟加载造成两个sql不同，所以后面。color_lot.color_lot_manuallies再次使用时，不会使用缓存的sql。
改成这样：

    color_lot_manuallies = color_lot.color_lot_manuallies.all
    if color_lot_manuallies.size == 1
    end
    color_lot.color_lot_manuallies.each do |i|
       puts i.id
    end

引用

ColorLotManually Load (0.1ms)   SELECT * FROM `color_lot_manuallies` WHERE (`color_lot_manuallies`.color_lot_id = 198)
CACHE (0.0ms)   SELECT * FROM `color_lot_manuallies` WHERE (`color_lot_manuallies`.color_lot_id = 198)

加一个all，color_lot.color_lot_manuallies.all，这样的话，后面再次调用时，会使用rails的sql缓存。

#PurchaseOrder 
  has_one :purchase_order_marketing, :dependent => :destroy
  has_many :purchase_invoices, :dependent => :destroy,:through => :purchase_order_marketing
#PurchaseOrderMarketing
  has_many :purchase_invoices,:dependent => :destroy

PurchaseOrder.first.purchase_invoices.all(:select => 'purchase_invoices.id')

引用

PurchaseOrder Load (0.2ms)   SELECT * FROM `purchase_orders` LIMIT 1
  PurchaseInvoice Load (0.8ms)   SELECT purchase_invoices.id FROM `purchase_invoices` INNER JOIN `purchase_order_marketings` ON `purchase_invoices`.purchase_order_marketing_id = `purchase_order_marketings`.id WHERE ((`purchase_order_marketings`.purchase_order_id = 13))

一条sql就完成了，真强大。

  #article
def sizes
    sizes = []
    art = self
    sf = art.article_secondary_feature
    size_group = SizeGroup.find_by_id(sf.size_groupid) if sf
    sizes = size_group.sizes if size_group
    return sizes
  end

ArticleMarketing.first.article.sizes.all(:select => 'id')

 

引用

ArticleMarketing Load (0.3ms)   SELECT * FROM `article_marketings` LIMIT 1
  Article Load (0.8ms)   SELECT * FROM `articles` WHERE (`articles`.`id` = 43)
  ArticleSecondaryFeature Load (0.7ms)   SELECT `article_secondary_features`.* FROM `article_secondary_features` WHERE (`article_secondary_features`.article_id = 43)
  SizeGroup Load (0.6ms)   SELECT * FROM `size_groups` WHERE (`size_groups`.`id` = 2) LIMIT 1
  Size Load (1.0ms)   SELECT id FROM `sizes` WHERE (`sizes`.size_group_id = 2)

find和find_by_id

params[:root].classify.constantize.find_by_id(params[:id])
params[:root].classify.constantize.find(params[:id])

引用

  Opportunity Load (0.1ms)   SELECT * FROM `opportunities` WHERE (`opportunities`.`id` = '126') LIMIT 1
  Opportunity Load (0.1ms)   SELECT * FROM `opportunities` WHERE (`opportunities`.`id` = 126)

find_by_id的SQL多了一行limit 1，对于sql语句而言，有一点性能上的提高。不过有时候我们需要find来捕捉异常。如果能用find_by_id最好了。
都改成find_by_id

引用

Opportunity Load (0.1ms)   SELECT * FROM `opportunities` WHERE (`opportunities`.`id` = '126') LIMIT 1
  CACHE (0.0ms)   SELECT * FROM `opportunities` WHERE (`opportunities`.`id` = '126') LIMIT 1

这样都有缓存了。rails的自带的缓存是很脆弱的，B.find_by_id(a.id)方法变成a.b时，这个缓存就不会用上。同样前面的例子里改成

    color_lot_manuallies = color_lot.color_lot_manuallies.all
    ColorLotManually.find_all_by_color_lot_id(color_lot.id)

rails自带的缓存也不会用上。
关于||=缓存，参考
http://fuliang.iteye.com/blog/827321
http://www.iteye.com/topic/810957
但是有一点，||=不会自动清除或者更新，所以使用的时候还是要注意点，可能会引起取值错误，而且不会报错。

class PortOfDischage < ActiveRecord::Base
  def _name
    @_name ||= self.city
  end
end

Reloading...
=> true
>> p=PortOfDischage.first
  SQL (0.2ms)   SET SQL_AUTO_IS_NULL=0
  PortOfDischage Load (21.2ms)   SELECT * FROM `port_of_dischages` LIMIT 1
  PortOfDischage Columns (1.7ms)   SHOW FIELDS FROM `port_of_dischages`
+----+------------+-----------------+---------+---------+--------------------------------+--------------------------------+
| id | ap_list_id | ap_marketing_id | city    | country | created_at                     | updated_at                     |
+----+------------+-----------------+---------+---------+--------------------------------+--------------------------------+
| 1  | 2          | 2               | Piraeus | Greece  | Thu Oct 07 07:10:21 +0800 2010 | Thu Oct 07 07:10:21 +0800 2010 |
+----+------------+-----------------+---------+---------+--------------------------------+--------------------------------+
1 row in set
>> p._name
=> "Piraeus"
>> p.update_attribute(:city,'p')
  SQL (0.2ms)   BEGIN
  ApMarketing Columns (43.0ms)   SHOW FIELDS FROM `ap_marketings`
  ApMarketing Load (18.6ms)   SELECT * FROM `ap_marketings` WHERE (`ap_marketings`.`id` = 2) 
  PortOfDischage Update (45.5ms)   UPDATE `port_of_dischages` SET `updated_at` = '2010-12-30 13:08:47', `city` = 'p' WHERE `id` = 1
  SQL (55.5ms)   COMMIT
=> true
>> p._name
=> "Piraeus"
>> p.city
=> "p"
>> p.reload
  PortOfDischage Load (0.7ms)   SELECT * FROM `port_of_dischages` WHERE (`port_of_dischages`.`id` = 1) 
+----+------------+-----------------+------+---------+--------------------------------+--------------------------------+
| id | ap_list_id | ap_marketing_id | city | country | created_at                     | updated_at                     |
+----+------------+-----------------+------+---------+--------------------------------+--------------------------------+
| 1  | 2          | 2               | p    | Greece  | Thu Oct 07 07:10:21 +0800 2010 | Thu Dec 30 13:08:47 +0800 2010 |
+----+------------+-----------------+------+---------+--------------------------------+--------------------------------+
1 row in set
>> p._name
=> "Piraeus"