使用has_many关系会导致其他MySQL查询吗？

Question

I'm going through the Lynda.com course "Ruby on Rails 4 Essential Training". It uses the creation of a simple CMS to teach the framework. In this CMS, users can create Subjects and Pages . Each Subject has many Pages . When clicking a link to view the pages that belong to a particular subject, it passes the subject id as a parameter to the Pages controller . The pages controller then does the following to determine which pages to show:

@subject = Subject.find(params[:subject_id])
@pages = @subject.pages

This simple, makes sense, and results in very understandable code. However, it results in an extra DB call by first loading the Subject , and then finding its pages. In the console I see:

  Subject Load (0.3ms)  SELECT  `subjects`.* FROM `subjects` WHERE `subjects`.`id` = 3 LIMIT 1
  Page Load (0.3ms)  SELECT `pages`.* FROM `pages` WHERE `pages`.`subject_id` = 3  ORDER BY pages.position ASC

It seems to me that the more efficient approach would be to write something like:

@pages = Page.where(subject_id: params[:subject_id])

Indeed, I see only a single SQL query when I do that. Is it considered best practice to use the first approach, despite the fact that it results in 2 queries? Or is that just being used for learning purposes, and ultimately my approach would be considered the better of the two?

Answer 1

It depends first whether you need both the @subject and the @pages object. You are right that your proposal is more performant, now this may or may not make a real difference for your user - you'll need profiling to be sure.

Finally, it is possible to make a slight change to the tutorial code to make it as performant using 'includes':

@subject = Subject.includes(:pages).find(params[:subject_id])
@pages = @subject.pages

This will load everything with a single query.

Answer 2

如果您不希望使用@subject或基本上是主题记录，则无需使用前者，则可以使用在以后的方法中编写的内容直接访问页面。