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施，这非常明显......

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

获得All C for A的最佳方式是什么？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。

class C需要与D

的关联

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

如果您想访问A的{{1}}，则还需要指定此内容。
```
D
```

现在，要访问class A < ActiveRecord::Base has_many :B has_many :C, :through => :B has_many :D, :through => :C end A的所有内容：

注意调用-> 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">]时如何不执行其他查询。这是因为ActiveRecord知道您希望通过a.C调用访问找到的A C，并生成最少数量的查询。同样适用于include＆＃39; s：

假设您想要所有-> 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) A，但希望D订购：

注意您也可以将其设置为关联的默认值：

A.where(:id => 1).includes(:C).order('cs.created_at DESC').includes(:D)选项指示将接收关联对象的顺序（使用SQL :order子句使用的语法）。
ORDER BY

高效的连接查询 - 可以使用ActiveRecord完成

1 个答案: