Ruby on Rails—Lecture 2

Hi there,

I’m Thomas. Welcome to the Ruby on Rails Lecture 2.

In this lecture, we learn the 1-to-many associations. We create a comment resource which is nested inside each post record.

After this lecture, you will learn:

Nested resources
1-to-many association
More understandings on the routes→controller↔model→view flow

I have recorded the implementation of the post-comment example in the following screencast.

Note: There is no narration for the video at the moment.

The steps for this project example:

First, we would like to generate the scaffold for Comment. We will heavily modify the generated code to fit into the post-comments association.

$ rails generate scaffold Comment author content:text
$ rails generate migration AddPostToComment post:references
$ rake db:migrate

The command generates the following migration files:

class CreateComments < ActiveRecord::Migration
  def change
    create_table :comments do |t|
      t.string :author
      t.text :content

      t.timestamps null: false
    end
  end
end

And the post_id column for the post reference:

class AddPostToComment < ActiveRecord::Migration
  def change
    add_reference :comments, :post, index: true, foreign_key: true
  end
end

Then you need to modify the following code:

In the routes.rb file, we want to make URL like: /posts/:post_id/comments/:id. We can do that by setting the routes to the following:

resources :posts do
  resources :comments, except: [:new]
end

The resources means we want a Restful CRUD route on the given resource. resources :posts essentially means defining the following 7 routes.

get 'posts' => 'posts#index'
get 'posts/new' => 'posts#new', as: 'new_post'
get 'posts/:id' => 'posts#show', as: 'post'
get 'posts/:id/edit' => 'posts#edit', as: 'edit_post'
post 'posts' => 'post#create'
patch 'posts/:id' => 'posts#update'
put 'posts/:id' => 'posts#update'
delete 'posts/:id' => 'posts#destroy'

You can see 8 lines here, which the put and patch is the same thing with an HTTP verb alias.

We need to describe the relationship in model. In the app/models/comment.rb file, we define a belongs_to:

class Comment < ActiveRecord::Base
  belongs_to :post
end

In the app/models/post.rb file, we define a has_many:

class Post < ActiveRecord::Base
  has_many :comments
end

We can test the model association in rails console:

$ rails console
> c = Comment.new
> c.author = 'Sam’
> c.content = 'Hi there' 
> c.post = Post.first    # assign the post to the first post.
> c.save                 # it won’t commit to the database until we call save.
> 
> # a better approach:
> p = Post.first
> p.comments.count       # result: 1
> c = p.comments.new     # post_id of comment is auto assigned.
> c.author = 'Sam'
> c.content = 'Hi there’
> c.save

We want to display the comments when show a post. The posts#show is in charged to show a single post. In the app/views/posts/show.html.erb file, we append the following to the end of the HTML code.

<h2>Comments</h2>

<table>
  <thead>
    <tr>
      <th>Author</th>
      <th>Content</th>
      <th colspan="3"></th>
    </tr>
  </thead>

  <tbody>
    <% @post.comments.each do |comment| %>
      <tr>
        <td><%= comment.author %></td>
        <td><%= comment.content %></td>
      </tr>
    <% end %>
  </tbody>
</table>

Further improvement

p.s. Next improvement suggestions:

Display post title in posts/edit view. e.g.
Editing {post title here}
Link the post title back to the post.
Delete un-used views in comments folder.
Improve the HTML code in view, and CSS files is assets folder, to create your personal blog style.

—Thomas

Ruby on Rails—Lecture 2

Further improvement

Editing {post title here}