Rails 4 ActiveRecord Includes

Question

I am trying to understand ActiveRecord includes. I am trying to retrieve both Folders and Grids in a single query. I have associated Grids and Folders in ActiveAdmin. Why does this query not return Grid associated data? How do I get Folder plus Grid associated data?

irb(main):002:0> Folder.includes(:grids)
      Folder Load (85.4ms)  SELECT `folders`.* FROM `folders`
      Grid Load (93.1ms)  SELECT `grids`.* FROM `grids` WHERE `grids`.`folder_id` IN (1, 2, 3)
    => #<ActiveRecord::Relation [#<Folder id: 1, name: "Red", created_at: "2014-03-14 06:02:09", updated_at: "2014-03-14 06:03:46">, #<Folder id: 2, name: "Yellow", created_at: "2014-03-14 06:03:33", updated_at: "2014-03-14 06:04:02">, #<Folder id: 3, name: "Blue", created_at: "2014-03-14 06:04:20", updated_at: "2014-03-14 06:04:20">]>

class Grid < ActiveRecord::Base
  belongs_to :folder
  has_many :role_grids
  has_many :roles, :through => :role_grids
end

class Folder < ActiveRecord::Base
  has_many :grids
end

schema:

create_table "folders", force: true do |t|
    t.string   "name"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "grids", force: true do |t|
    t.string   "title"
    t.string   "description"
    t.datetime "created_at"
    t.datetime "updated_at"
    t.integer  "folder_id"
  end

Answer 1

You are actually retrieving all of the Grid data. If you were to check any of the folders for their grids, they would all be there without executing any more SQL queries.

The point of using includes is to eager load associations, avoiding the n+1 queries problem.

A good write up on eager loading, and why n+1s are bad, is here:

http://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations