Question

I have 4 models, A, B, C and 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    

I have a very naive implementation which is very obvious …

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

What’s the best way to get All C for A?
What’s the best way get All D for A?

I want to get all ‘C’ using order_by clause with “created_at” value instead of iterating through B.

May be I’m missing some ActiveRecord magic?

I appreciate any help.

Answer 1

First of all, you need to make a couple changes.

1. class C needs an association to D

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

2. If you want to access A‘s D‘s, you need to specify this as well.

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

Now, to access all of A‘s C‘s:

-> 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">]

Notice how another query is not executed when you call a.C. This is because ActiveRecord knows you will want to access the found A‘s C‘s by the include call, and generates the minimum number of queries. Same goes for D‘s:

-> 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)

Say you wanted all A‘s D‘s but wanted C‘s ordered:

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

Note you can also set this as a default on the association:

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