Rails Association Undefined method error - ruby-on-rails

Question

I would like to add "category" function.
I associated article.rb and category.rb.
However, undefined method `categories' for nil:NilClass was present.
I have no idea.If you know any solution, please tell me.
Index.html.erb
<% unless #article.categories.blank? %>
<% #articles.categories.each do |category|%>
<%= link_to category.name,article_path(category_id:category.id)%>
<%end%>
<%end%>
article.rb
class Article < ApplicationRecord
scope :from_category, -> (category_id) { where(id: article_ids = ArticleCategory.where(category_id: category_id).select(:article_id))}
validates :title, presence: true
validates :content, presence: true
mount_uploader :image,ImageUploader
has_many :categories, through: :article_categories
has_many :article_categories, dependent: :destroy
def save_categories(tags)
current_tags = self.categoires.pluck(:name) unless self.categories.nil?
old_tags = current_tags - tags
new_tags = tags - current_tags
old_tags.each do |old_name|
self.categories.delete Category.find_by(name:old_name)
end
new_tags.each do |new_name|
article_category = Category.find_or_create_by(name:new_name)
self.categories << article_category
end
end
end
category.rb
class Category < ApplicationRecord
validates :name,presense: true,length:{maximum:50}
has_many :articles,through: :article_categories
has_many :article_categories,dependent: :destroy
end
article_category.rb
class ArticleCategory < ApplicationRecord
belongs_to :article
belongs_to :category
validates :article_id,presense:true
validates :category_id,presense:true
end
articles_controller.rb
class ArticlesController < ApplicationController
before_action :set_post, only: [:show,:edit,:update]
before_action :authenticate_user!, :except => [:index,:show]
before_action :set_article_tags_to_gon, only: [:edit]
def index
if params[:category_id]
#selected_category = Category.find(params[:category_id])
#articles= Article.from_category(params[:category_id]).page(params[:page])
else
#articles= Article.all.page(params[:page])
end
#articles = Article.page params[:page]
end
def show
end
def new
#article = Article.new
end
def create
#article = Article.new(article_params)
if #article.save
redirect_to articles_path
else
render 'articles/new'
end
end
def edit
#category_list = #article.categories.pluck(:name).join(",")
end
def update
if #article.update(article_params)
redirect_to articles_path
else
redirect_to 'edit'
end
end
private
def article_params
params[:article].permit(:title,:content,:image,:tag_list,:category)
end
def set_post
#article = Article.find(params[:id])
end
error message

Answer 1

You're calling categories on a nil object, in this case #article. Did you mean to call it on #articles?
If not, you will need to define #article in the index action of your controller.