连接表时，rails 在访问连接表中的字段时无论如何都会发出额外的请求

我有公司表和城市表；公司属于城市。

我的数据库架构的一部分是：

  create_table "companies", force: true do |t|
    t.string   "title",      default: "", null: false
    t.string   "address",    default: "", null: false
    t.integer  "city_id"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "cities", force: true do |t|
    t.string   "title",      default: "", null: false
    t.datetime "created_at"
    t.datetime "updated_at"
    t.string   "test"
  end

Part of Company model:

class Company < ActiveRecord::Base
  belongs_to    :city,   :inverse_of => :companies
  # ....
end

Part of City model:

class City < ActiveRecord::Base
  has_many      :companies, :inverse_of => :city
  # ....
end

当我获取公司，然后获取其城市标题时，很明显 Rails 会发出额外的请求来获取城市标题：

$ rails console
2.1.5 :001 > Company.first.city.title
  Company Load (0.2ms)  SELECT  "companies".* FROM "companies"   ORDER BY "companies"."id" ASC LIMIT 1
  City Load (0.3ms)  SELECT  "cities".* FROM "cities"  WHERE "cities"."id" = ? LIMIT 1  [["id", 2]]
 => "My city 1"

我很确定我可以通过加入来避免这种情况cities桌子。但是我错了：

2.1.5 :002 > Company.joins(:city).first.city.title
  Company Load (0.3ms)  SELECT  "companies".* FROM "companies" INNER JOIN "cities" ON "cities"."id" = "companies"."city_id"  ORDER BY "companies"."id" ASC LIMIT 1
  City Load (0.1ms)  SELECT  "cities".* FROM "cities"  WHERE "cities"."id" = ? LIMIT 1      [["id", 2]]
 => "My city 1" 

So, cities表已加入，但 Rails 无论如何都会执行额外的请求cities.

这是为什么，以及如何避免额外的请求cities table?

当我只有一条记录时（如上例所示），这并没有多大影响，但如果我想得到Company.joins(:city).all，然后获取每个公司的城市，然后每次获取的额外开销cities is very重要的。

Try includes as in Company.includes(:city).first.city.title.