Rails - undefined method `username' for nil:NilClass

Question

I'm making a website and it has articles and comments too, articles are not associated with users because only I as an admin can create articles...But there are the comments that are associated with users and articles too but when I try to show the username of the user that create the comment, in the comment section of an article it give me this error "undefined method `username' for nil:NilClass"

Models

User model

has_many :comments

Article model

has_many :comments

Comment model

  belongs_to :article
  belongs_to :user

Articles Controller where comments are shown

def show
    @article = Article.find(params[:id])
    @comments = @article.comments
end

and the view

<h2>Comments <%= @comments.count %></h2>
    <% @comments.each do |comment| %>
        <%= comment.content %>
        <%= comment.user.username %>
    <% end %>

Comments controller where comment is created

def create
    @article = Article.find(params[:article_id])
    @comment = Comment.new(comment_params)
    @comment.article = @article
    @comment.user = current_user
    @comment.save

    redirect_to article_path(@article)
end

Answer 1

You'd need a setup like the following:

# app/models/user.rb
class User < ActiveRecord::Base
  has_many :articles
  has_many :comments, through: :articles
end

# app/models/article.rb
class Article < ActiveRecord::Base
  belongs_to :user
  has_many :comments, dependent: :destroy
end

# app/models/comment.rb
class Comment < ActiveRecord::Base
  belongs_to :article
  belongs_to :user
end

rails g migration AddUsernameToUsers username:string:uniq
rails g migration AddUserIdToArticles user_id:integer
rails g migration AddArticleIdToComments article_id:integer
rails g migration AddUserIdToComments user_id:integer
rake db:migrate

This allows you to have a .username method you can call on the User model, and allows for the following relation to return a proper username:

u = User.new(:username => "Foo")
u.save
a = Article.new(:user_id => 1)
a.save
comment = Comment.new(:article_id => 1, :user_id => 1)
comment.save

comment.user.username
# => "Foo"

Please note that if you're using Devise, you'd need to do something like:

rails g migration AddUsernameToUsers username:string:uniq
rake db:migrate