Rails - NoMethodError - undefined method `name' for nil:NilClass

Question

I'm new to rails and recently noticed an error on one of my show views while working on a project for school. The error has to do with the name method not being defined yet I'm unsure how to remedy this. Help would be much appreciated! The error I receive is:

NoMethodError in Topics#show

Showing /Users/Jason/code/bloccit/app/views/topics/show.html.erb where line #17 raised:

undefined method `name' for nil:NilClass

<%= link_to post.title, [@topic, post] %>
         </h4>
         <small>
           submitted <%= time_ago_in_words(post.created_at) %> ago by <%= post.user.name %><br>
           <%= post.comments.count %> Comments
         </small>
       </div>

Relevant files include..

topics/show.html.erb

<h1><%= @topic.name %></h1>

<% if policy(@topic).update? %>
  <%= link_to "Edit Topic", edit_topic_path, class: 'btn btn-success' %>
<% end %>

<div class="row">
  <div class="col-md-8">
    <p class="lead"><%= @topic.description %></p>
    <% @posts.each do |post| %>
      <div class="media">
        <div class="media-body">
          <h4 class="media-heading">
            <%= link_to post.title, [@topic, post] %>
          </h4>
          <small>
            submitted <%= time_ago_in_words(post.created_at) %> ago by <%= post.user.name %><br>
            <%= post.comments.count %> Comments
          </small>
        </div>
      </div>
    <% end %>
  </div>
  <div class="col-md-4">
    <% if policy(Post.new).create? %>
      <%= link_to "New Post", new_topic_post_path(@topic), class: 'btn btn-success' %>
    <% end %>
  </div>
</div>

post.rb

class Post < ActiveRecord::Base
  has_many :comments
  belongs_to :user
  belongs_to :topic

  default_scope { order('created_at DESC') }
end

topic.rb

class Topic < ActiveRecord::Base
  has_many :posts
end

posts_controller.rb

class PostsController < ApplicationController
  def show
    @post = Post.find(params[:id])
    @topic = Topic.find(params[:topic_id])
    authorize @post
  end

  def new
    @topic = Topic.find(params[:topic_id])
    @post = Post.new
    authorize @post
  end

  def create
    @topic = Topic.find(params[:topic_id])
    @post = Post.new(post_params)
    @post.topic = @topic
    authorize @post

    if @post.save
      flash[:notice] = "Post was saved."
      redirect_to [@topic, @post]
    else
      flash[:error] = "There was an error saving the post.  Please try again."
      render :new
    end
  end

  def edit
    @topic = Topic.find(params[:topic_id])
    @post = Post.find(params[:id])
    authorize @post
  end

  def update
    @topic = Topic.find(params[:topic_id])
    @post = Post.find(params[:id])
    authorize @post

    if @post.update_attributes(post_params)
      flash[:notice] = "Post was updated."
      redirect_to [@topic, @post]
    else
      flash[:error] = "There was an error saving the post.  Please try again."
      render :new
    end
  end

  private

  def post_params
    params.require(:post).permit(:title, :body)
  end
end

topics_controller.rb

class TopicsController < ApplicationController
  def index
    @topics = Topic.all
    authorize @topics
  end

  def new
    @topic = Topic.new
    authorize @topic
  end

  def show
    @topic = Topic.find(params[:id])
    @posts = @topic.posts
    authorize @topic
  end

  def create
    @topic = Topic.new(topic_params)
    authorize @topic
    if @topic.save
      redirect_to @topic, notice: "Topic was saved successfully."
    else
      flash[:error] = "Error creating topic.  Please try again."
      render :new
    end
  end

  def edit
    @topic = Topic.find(params[:id])
    authorize @topic
  end

  def update
    @topic = Topic.find(params[:id])
    authorize @topic
    if @topic.update_attributes(topic_params)
      redirect_to @topic, notice: "Topic was updated successfully."
    else
      flash[:error] = "Error saving topic.  Please try again."
      render :edit
    end
  end

  private

  def topic_params
    params.require(:topic).permit(:name, :description, :public)
  end
end

Answer 1

you are not setting the user when creating a post, Im not sure about the implementation of your authorize method. However,

it should be something like

#assuming you have the user as `current_user`
class PostsController < ApplicationController
  ...
  def create
    @topic = Topic.find(params[:topic_id])
    @post = Post.new(post_params)
    @post.user = current_user
    @post.topic = @topic
    ...
  end 
  ...
end

Answer 2

your post has no user. you have to check why your post not saved the user. to prevent this error you have to check, user exist in post.

i would prefer to use delegate and follow the law of demeter

post.rb

class Post < ActiveRecord::Base
  has_many :comments
  belongs_to :user
  belongs_to :topic

  delegate :name, :to :user, allow_nil: true, prefix: true
  default_scope { order('created_at DESC') }
end

after this, you can receive the users name from the post like post.user_name and it will be forwarded to the name attribute of the user object.

and change in your show.html

<%= post.user.name %>

to

<%= post.user_name %>

additional: please do not use default_scope. if you dont need the order() you always have to unscope your queries.