Rails: Routing "Courses" -> "Chapters" -> "Lessons" - ruby-on-rails

Question

im new to Ruby on Rails. I try to connect 3 tables.
My Models:
Course.rb:
class Course < ApplicationRecord
has_many :chapters
has_many :lessons, through: :chapters
end
Chapter.rb
class Chapter < ApplicationRecord
belongs_to :course
belongs_to :lesson
end
Lesson.rb
class Lesson < ApplicationRecord
has_many :chapters
has_many :courses, through: :chapters
end
My Course Controller looks like this:
class CourseController < ApplicationController
def index
end
def show
#course = Course.find(params[:id])
#chapters = #course.chapters
#lessons = #course.lessons
end
end
and show.html.erb
<% #chapters.each do |c| %>
<%= c.chapter %>
<%= link_to "lektionen", c %>
<% end %>
I can see a list of my courses. Thats working. Also i can see the chapters. But for any reason it is not forwarding the id for making a relationship between these models.
Chapter Controller:
class ChapterController < ApplicationController
def index
end
def show
#chapters = Chapter.find(params[:id])
#lessons = #chapters.lessons
end
end
i want to forward the id of the course so that chapters << lessons are the children of courses.
-courses
-- chapters
--- lessons
My schema:
ActiveRecord::Schema.define(version: 2021_04_11_090326) do
create_table "chapters", force: :cascade do |t|
t.string "chapter"
t.integer "course_id", null: false
t.integer "lesson_id", null: false
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.index ["course_id"], name: "index_chapters_on_course_id"
t.index ["lesson_id"], name: "index_chapters_on_lesson_id"
end
create_table "courses", force: :cascade do |t|
t.string "name"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
end
create_table "lessons", force: :cascade do |t|
t.string "title"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
end
add_foreign_key "chapters", "courses"
add_foreign_key "chapters", "lessons"
end
My routes.rb
Rails.application.routes.draw do
get 'chapter/index'
get 'chapter/show'
get 'course/index'
get 'course/show'
# For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
resources :course
resources :chapter
resources :lessons
end
if i click "courses#show" action i could see the chapters. If i click on one chapter i got this error:
undefined method `lessons' for #Chapter:0x000055f7aeefbe50
I want to show only lessons which are belongs to a specific chapter and the chapter is related to the course.
But im not in able to get this working.
I hope yo can help me.

Answer 1

I think you should rethink you data model in this case:
Make it Course -> Chapter -> Lesson
class Course < ApplicationRecord
has_many :chapters # a course has many chapters. alright
end
class Chapter < ApplicationRecord
belongs_to :course # a chapter always belongs to a course
has_many :lessons # and has many lessons ! check!
end
class Lesson < ApplicationRecord
belongs_to :chapter # a lesson belongs to a chapter
# helper method if you want to access course directly from lesson
def course
#course ||= chapter&.course
end
end
Database
Your database should look something like this:
suggestion -> do not name it chapter in chapters table. call it title or name
ActiveRecord::Schema.define(version: 2021_04_11_090326) do
create_table "courses", force: :cascade do |t|
t.string "name"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
end
create_table "chapters", force: :cascade do |t|
t.string "title"
t.integer "course_id", null: false
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.index ["course_id"], name: "index_chapters_on_course_id"
end
create_table "lessons", force: :cascade do |t|
t.string "title"
t.integer "chapter_id", null: false
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.index ["chapter_id"], name: "index_lessons_on_chapter_id"
end
end
With this setup you are ready to go with a good hierarchy.
Draw the routes:
Rails.application.routes.draw do
resources :courses
resources :chapters
resources :lessons
end
if you want to make the hierarchy in the url you could do something more advanced
Rails.application.routes.draw do
# use this if you know what you are doing :D
resources :courses do
resources :chapters do
resources :lessons
end
end
end
Your controllers:
class CourseController < ApplicationController
def index
courses
end
def show
course
end
protected
# get all courses
def courses
#courses ||= Course.order(:name)
end
# find course by id
def course
#course ||= courses.find(params[:id])
end
end
In your view views/courses/show.html.erb
<h1><%= #course.name %></h1>
<% #course.chapters.each do |chapter| %>
<h2><%= chapter.title %></h2>
<% chapter.lessons.each do |lesson| %>
<%= link_to "Lesson: " + lesson.title, lesson_url(lesson) %>
<% end %>
<% end %>
N+1 problem:
To avoid N+1 problem when rendering this hierarchy change your controller and add includes to the query:
def course
#course ||= courses.includes(chapters: :lessons).find(params[:id])
end
I hope this gives you a good start with a clean data model.