ActiveRecord::Associations::CollectionProxy

Rails4では、has_many関連の中身はActiveRecord::Associations::CollectionProxyが入っている。
ドキュメント
http://api.rubyonrails.org/classes/ActiveRecord/Associations/CollectionProxy.html

app/model/user.rb

class User < ActiveRecord::Base
  has_many :comments
end

app/model/comment.rb

class Comment < ActiveRecord::Base
  belongs_to :user
end

db/schema.rb

create_table "comments", force: true do |t|
  t.integer  "user_id"
  t.datetime "created_at"
  t.datetime "updated_at"
end

create_table "users", force: true do |t|
  t.string   "comment"
  t.datetime "created_at"
  t.datetime "updated_at"
end

irb(main):001:0> user = User.create
irb(main):002:0> user.comments.create
irb(main):004:0> p user.comments
#<ActiveRecord::Associations::CollectionProxy [#<Comment id: 8, user_id: 2, created_at: "2013-07-16 07:58:18", updated_at: "2013-07-16 07:58:18">]>

#中身を取り出すときはtarget
irb(main):005:0> p user.comments.target
[#<Comment id: 8, user_id: 2, created_at: "2013-07-16 07:58:18", updated_at: "2013-07-16 07:58:18">]
=> [#<Comment id: 8, user_id: 2, created_at: "2013-07-16 07:58:18", updated_at: "2013-07-16 07:58:18">]

#数をかぞえるときはcount。毎回SQLが発行される
irb(main):006:0> p user.comments.count
=> 1

#sizeでも数えられる。sizeは一回ロードすればSQLが発行されないっぽい
irb(main):007:0> p user.comments.size
=> 1