高效的连接查询 - 可以使用ActiveRecord完成

Question

我有4个型号,A,B,C和D.

class A < ActiveRecord::Base
  has_many :B
  has_many :C, :through => :B
end  

class B < ActiveRecord::Base
  belongs_to :A  
  has_many   :C
  has_many   :D, :through => :C
end  

class C < ActiveRecord::Base    
  belongs_to :B
end    

class D < ActiveRecord::Base    
  belongs_to :C
end    

我有一个非常天真的实现,非常明显......

<% A.B.each do |b| %>
  <%= b.number %>
  <% b.C.each do |c| %>
    <%= c.name %>
  <% end %>
<% end %>

获得All C for A的最佳方法是什么？获得All D for A的最佳方式是什么？

我希望使用带有"created_at"值的order_by子句而不是迭代B来获取所有'C'.

可能是我错过了一些ActiveRecord魔法？

我感谢任何帮助.

Answer 1

首先,您需要进行一些更改.

1. class C 需要一个关联 D

   class C < ActiveRecord::Base
     belongs_to :B
     has_one :D
   end
   

2. 如果你想访问A's D',你也需要指定它.

   class A < ActiveRecord::Base
     has_many :B
     has_many :C, :through => :B
     has_many :D, :through => :C
   end
   

现在,访问所有A的C的:

-> a = A.where(:id => 1).includes(:C).first
  A Load (0.2ms)  SELECT "as".* FROM "as" WHERE "as"."id" = 1 LIMIT 1
  B Load (0.1ms)  SELECT "bs".* FROM "bs" WHERE "bs"."a_id" IN (1)
  C Load (0.1ms)  SELECT "cs".* FROM "cs" WHERE "cs"."b_id" IN (1, 2)
 => #<A id: 1, created_at: "2012-01-10 04:28:42", updated_at: "2012-01-10 04:28:42"> 
-> a.C
 => [#<C id: 1, b_id: 1, created_at: "2012-01-10 04:30:10", updated_at: "2012-01-10 04:30:10">, #<C id: 2, b_id: 1, created_at: "2012-01-10 04:30:11", updated_at: "2012-01-10 04:30:11">, #<C id: 3, b_id: 2, created_at: "2012-01-10 04:30:21", updated_at: "2012-01-10 04:30:21">, #<C id: 4, b_id: 2, created_at: "2012-01-10 04:30:21", updated_at: "2012-01-10 04:30:21">]

请注意调用时如何不执行另一个查询a.C.这是因为ActiveRecord知道您将要通过调用访问找到A的,并生成最少数量的查询.同样适用于:CincludeD

-> a = A.where(:id => 1).includes(:D).first
  A Load (0.1ms)  SELECT "as".* FROM "as" WHERE "as"."id" = 1 LIMIT 1
  B Load (0.1ms)  SELECT "bs".* FROM "bs" WHERE "bs"."a_id" IN (1)
  C Load (0.1ms)  SELECT "cs".* FROM "cs" WHERE "cs"."b_id" IN (1, 2)
  D Load (0.1ms)  SELECT "ds".* FROM "ds" WHERE "ds"."c_id" IN (1, 2, 3, 4)

假设你希望所有A的D的,但希望C的命令:

A.where(:id => 1).includes(:C).order('cs.created_at DESC').includes(:D)

请注意,您还可以将此设置为关联的默认值:

该:order选项指示将接收关联对象的顺序(在SQL ORDER BY子句使用的语法中).

class Customer < ActiveRecord::Base
  has_many :orders, :order => "date_confirmed DESC"
end