Rails view rendering takes too long time with fetching child object in DB

Question

My rails application took so long time to render the view. This view has consisted of child objects.

First of all, My 'course' object belong to 'school'

class Course < ApplicationRecord
    belongs_to :school
end

class School < ApplicationRecord
    has_many :courses, dependent: :destroy 
end

and Controller.

class CoursesController < ApplicationController
    before_action :set_course , only: [:show, :edit, :update, :destroy]

    # GET /courses
    # GET /courses.json
    def index
       @courses = Course.all
    end

   // other stuffs..

end

In a View, I have a part for rendering the course's relation with a school in DB.

index.html.erb

<div>
    <table>
        <thead>
            //.....stuffss..
        </thead>
        <tbody>

            <% @courses.each do |course| %>
            <tr>
                <th><%= course.school.location %></th> // This parts are referencing 'school'..
                <td><%= course.school.name %></td>
                <td><%= course.name %></td>
                <td><%= convert_currency(course.price) %></td>
                <td><%= link_to "course", course_path(course.id), :class => 'button'  %></td>
                <td><%= course.school.nationalmix %></td>
                <td><%= course.school.certificate %></td>
                <td><%= course.school.good %></td>
                <td><%= course.school.bad %></td>
                <td><%= link_to "course", course_path(course.id), :class => 'button'  %> </td>
            </tr>
            <% end %>

        </tbody>
    </table>
</div>

It turned out that rendering page is taking so long time.

Started GET "/courses" for ::1 at 2019-11-21 10:57:23 +0000
Processing by CoursesController#index as HTML
  Rendering courses/index.html.erb within layouts/application
  Course Load (0.3ms)  SELECT "courses".* FROM "courses"
  ↳ app/views/courses/index.html.erb:102
  School Load (0.2ms)  SELECT "schools".* FROM "schools" WHERE "schools"."id" = $1 LIMIT $2  [["id", 4], ["LIMIT", 1]]
  ↳ app/views/courses/index.html.erb:104
  School Load (0.2ms)  SELECT "schools".* FROM "schools" WHERE "schools"."id" = $1 LIMIT $2  [["id", 5], ["LIMIT", 1]]
  ↳ app/views/courses/index.html.erb:104
  School Load (0.4ms)  SELECT "schools".* FROM "schools" WHERE "schools"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  ↳ app/views/courses/index.html.erb:104
  CACHE School Load (0.0ms)  SELECT "schools".* FROM "schools" WHERE "schools"."id" = $1 LIMIT $2  [["id", 1], ["LIMIT", 1]]
  ↳ app/views/courses/index.html.erb:104
  School Load (0.3ms)  SELECT "schools".* FROM "schools" WHERE "schools"."id" = $1 LIMIT $2  [["id", 2], ["LIMIT", 1]]
  ↳ app/views/courses/index.html.erb:104
  Rendered courses/index.html.erb within layouts/application (Duration: 7941.3ms | Allocations: 184810) // See this line.
  User Load (0.4ms)  SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 6], ["LIMIT", 1]]
  ↳ app/views/layouts/application.html.erb:71
Completed 200 OK in 8015ms (Views: 8011.2ms | ActiveRecord: 1.8ms | Allocations: 205787)

Rendered courses/index.html.erb within layouts/application (Duration: 7941.3ms | Allocations: 184810)

Almost 8 seconds. Is there any better way of doing this?

score 4 · Accepted Answer · answered Nov 21 '19 at 11:11

4

You need to load schools eagerly:

@courses = Course.all.includes(:school)

Very detailed description you can find in this excellent blog post

answered Nov 21 '19 at 11:11

mrzasa

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I applied this and other recommendations in the blog post. but it didn't improve the performances. – Jin Lim Nov 21 '19 at 11:48
Do you still have multiple SQL statements for loading schools in your logs? – mrzasa Nov 21 '19 at 11:51
Maybe you should paginate the results, instead of showing all of them on a single page? How many courses d you have? – mrzasa Nov 21 '19 at 11:52
I can see only 1 SQL statement. yes true. and I also assume that `convert_currency(course.price)` this making API call which caused the issue mainly. – Jin Lim Nov 21 '19 at 12:15
Yes, It was the problem. and I solved it with Rails Caching. and Thanks for the eager_loading method. That obviously helped performance somehow. https://stackoverflow.com/a/30111615/4735043 – Jin Lim Nov 21 '19 at 16:56

Rails view rendering takes too long time with fetching child object in DB

1 Answers1