undefined method `articles' for nil:NilClass when i tried to display related articles on my article show page

Question

Hello friends i have this error I've been facing yesterday on my Rails App. I get undefined method `articles' for nil:NilClass when i tried to display related articles on my article show page .

Here is my app codes

tag.rb

class Tag < ApplicationRecord
  has_many :taggings
  has_many :articles, through: :taggings

  def to_s
    name
  end

end

tagging.rb

class Tagging < ApplicationRecord
  belongs_to :tag
  belongs_to :article
end

articles_controller

class ArticlesController < ApplicationController
 before_action :find_article, only: [:show, :edit, :update, :destroy]
 before_action :owned_article, only: [:edit, :update, :destroy]
 before_action :authenticate_user!, except: [:index, :show]

def index
 @articles = Article.all.order("created_at desc")
end

def show
end

def new
  @article = current_user.articles.build
end

def create
  @article = current_user.articles.build(article_params)

  if @article.save
    redirect_to @article
  else
    render 'new'
  end
end

def edit
end

def update
  if @article.update(article_params)
    redirect_to @article, notice: "Your article was successfully updated!"
  else
    render 'edit'
  end
end

def destroy
  @article.destroy
  redirect_to articles_path
end

private

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

def article_params
  params.require(:article).permit(:title, :content, :image, :tag_list)
end

def owned_article
  unless current_user == @article.user
    flash[:alert] = "That article does not belong to you!"
    redirect_to root_path
  end
end

end

articles show.html.erb

<div class="container">
<div class="row text-white text-center">
    <div class="col-md-10 col-lg-10 ml-sm-auto mr-sm-auto article-show-col">
        <br>
        <h1><%= @article.title %></h1>
        <p class="text-muted">Posted on: <%=       @article.created_at.strftime('%-b %-d, %Y') %></p>

        <p>
          Tags:
          <% @article.tags.each do |tag| %>
            <%= link_to tag.name, tag_path(tag) %>
          <% end %>
        </p>


        <!-- <br> -->
        <div class="article-show-image">
            <%= image_tag @article.image.url(:wide) %>
        </div>
        <!-- <br> -->
        <p><%= @article.content.html_safe %></p>

        <hr class="index-hr">

        <h5>Broadcast this article</h5>

        <%= social_share_button_tag("Hey! Checkout this new article from TWM!") %>

        <hr class="index-hr">

        **<h5>Related Articles</h5>
        <% @tag.articles.each do |article| %>
            <li><%= link_to article.title, article_path(article) %></li>
        <% end %>**

        <div class="btn-group">
            <%= link_to "Back", articles_path, class: "btn-custom btn-sm" %>
            <% if user_signed_in? %>
                <% if @article.user_id == current_user.id %>
                    <%= link_to "Delete", article_path(@article), method: :delete, data: { confirm: "Are you sure you want to delete this article?" }, class: "btn-custom btn-sm" %>
                    <%= link_to "Edit", edit_article_path, class: "btn-custom btn-sm" %>
                <% end %>
            <% end %>
        </div>
    </div>
</div>

tags controller

class TagsController < ApplicationController
before_action :find_article, only: [:show, :edit, :update, :destroy]

def index
    @tags = Tag.all.order("created_at desc")
end

def show
end

def destroy
    @tag.destroy
    redirect_to tags_path
end

private

def find_article
    @tag = Tag.find(params[:id])
end
end

show view for tags

<div class="container text-white text-center">

<h1>Articles Tagged with <%= @tag.name %></h1>

<ul>
  <% @tag.articles.each do |article| %>
    <li><%= link_to article.title, article_path(article) %></li>
  <% end %>
</ul>

Thank You!

Answer 1

Here is a longer answer that offers a solution to your problem. The issue is that you want to get all articles that share a tag with the article you are showing, while presumably not showing the current article in the list of related articles. I would accomplish this by adding a related_articles method to your Article model and calling it in your view.

Add the following method to app/models/article.rb :

def related_articles
    Article.joins(:tags).where(tags: { id: self.tags.pluck(:id) }).where.not(id: self.id)
end

The above query should return all of the articles that have a matching tag while excluding itself.

You can now replace the related articles section in your view with:

**<h5>Related Articles</h5>
<% @article.related_articles.each do |article| %>
    <li><%= link_to article.title, article_path(article) %></li>
<% end %>**

One final note that is not strictly related to your problem, but worth mentioning. By iterating over @article.tags , your view is creating an N+1 query. These are very inefficient. The good news, is that this can be fixed with eager loading by simply, changing the find_articles method in your articles_controller as follows:

def find_article
  @article = Article.includes(:tags).find(params[:id])
end

There may be a more efficient way to write the related_articles query, but this should work.

EDIT:

Another way of writing the related_articles query follows. This will yield the same results. It moves more of the processing to the database and results in fewer calls to the database.

def related_articles
    Article.distinct.joins(tags: :articles).where.not(id: self.id)
end

Answer 2

您的 ArticlesController 在显示视图中使用时不会实例化 @tag 变量。

Answer 3

In your show.html.erb , you're trying to do:

<div class="container">
  <div class="row text-white text-center">
    <div class="col-md-10 col-lg-10 ml-sm-auto mr-sm-auto article-show-col">

      ...

      <% @tag.articles.each do |article| %>
          <li><%= link_to article.title, article_path(article) %></li>
      <% end %>**

      ...

    </div>
  </div>
</div>

But, hey, check it out! No @tag in your show action:

class ArticlesController < ApplicationController
  before_action :find_article, only: [:show, :edit, :update, :destroy]
  before_action :owned_article, only: [:edit, :update, :destroy]
  before_action :authenticate_user!, except: [:index, :show]

  ...

  def show
    #look! no @tag
  end

  ...

end