ruby-on-rails - Rails 急于加载所有记录但仍生成 N+1 查询

Question

我正在使用 Rails 4.1，并且有一个非常基本的问题让急切加载工作。

我将一个复杂的程序简化为一个非常基本的事件、用户和预订示例:

事件模型:

class Event < ActiveRecord::Base
  has_many :reservations

  def reserved?(user)
    reservations.where(user: user).present?
  end
end

用户模型:

class User < ActiveRecord::Base
  has_many :reservations
end

预订模型:

class Reservation < ActiveRecord::Base
  belongs_to :user
  belongs_to :event
end

我的事件 Controller :

def index
  @events = Event.includes(:reservations).all
  @user = User.find(1)
end

我的views/events/index.html.erb View :

<% @events.each do |event| %>
  <p>Event: <%= event.name %><br>
  Reserved?: <%= event.reserved?(@user) %><br></p>
<% end %>

我创建了一个用户和三个事件。事件索引页的结果服务器日志:

Started GET "/events" for 127.0.0.1 at 2014-12-03 11:26:20 +0800
Processing by EventsController#index as HTML
  User Load (0.1ms)  SELECT  "users".* FROM "users"  WHERE "users"."id" = ? LIMIT 1  [["id", 1]]
  Event Load (0.1ms)  SELECT "events".* FROM "events"
  Reservation Load (0.2ms)  SELECT "reservations".* FROM "reservations"  WHERE "reservations"."event_id" IN (1, 2, 3)
  Reservation Load (0.1ms)  SELECT "reservations".* FROM "reservations"  WHERE "reservations"."event_id" = ? AND "reservations"."user_id" = 1  [["event_id", 1]]
  Reservation Load (0.0ms)  SELECT "reservations".* FROM "reservations"  WHERE "reservations"."event_id" = ? AND "reservations"."user_id" = 1  [["event_id", 2]]
  Reservation Load (0.1ms)  SELECT "reservations".* FROM "reservations"  WHERE "reservations"."event_id" = ? AND "reservations"."user_id" = 1  [["event_id", 3]]
  Rendered events/index.html.erb within layouts/application (10.0ms)
Completed 200 OK in 76ms (Views: 68.7ms | ActiveRecord: 1.3ms)

如您所见，rails 正确地预先加载了我的事件的所有预订。但是，它会在 reserved?(user) 调用期间再次查询数据库。我究竟做错了什么？谢谢。

Answer 1

您的 reserved? 方法使用 reservations 关联的 where 方法。 where 方法是一个 ARel 方法，用于构建 SQL 查询。对 where 方法的结果调用 present? 会导致执行查询。

为了避免在 reserved? 中进行另一次数据库查找，您应该使用 Ruby 集合方法，例如:

def reserved?(user)
  reservations.select { |reservation| reservation.user == user }.empty?
end

或者，对于更简洁的版本(有人将其添加为评论，但看起来可能已被删除):

def reserved?(user)
  reservations.map(&:user_id).include?(user.id)
end