ruby-on-rails - 高效的连接查询 - 可以使用 ActiveRecord 来完成吗

Question

我有 4 个模型，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    

我有一个非常幼稚的实现，非常明显......

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

为 A 获得所有 C 的最佳方法是什么？
获得全 D 换 A 的最佳方法是什么？

我想使用带有“created_at”值的 order_by 子句来获取所有“C”，而不是通过 B 进行迭代。

可能是我错过了一些 ActiveRecord 魔法？

我很感激任何帮助。

Answer 1

首先，您需要进行一些更改。

class C需要关联到 D

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

如果您想访问 A的 D 's，你也需要指定这个。

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">]

注意当您调用 a.C 时，另一个查询是如何不被执行的。 .这是因为 ActiveRecord 知道您将要访问找到的 A的 C由 include 提供调用，并生成最少数量的查询。 D 也是如此的:

-> 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)

说你想要所有 A的 D的但想要 C的命令:

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

请注意，您还可以将其设置为关联的默认值:

The :order option dictates the order in which associated objects will be received (in the syntax used by an SQL ORDER BY clause).

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