开发日志：ActionView :: Template :: Error（未定义的方法`name'代表nil：NilClass）：

Question

I'm a newbie and this is for the Rails tutorial by Richard Schneeman. This is all that is in my index.html.erb file in my view/products folder.

<% first_product = Product.first %>
<% lots_of_products = Product.includes(:user).all %>
<ul>
  <% lots_of_products.each do |product| %>
    <li>
      Product Name: "<%= product.name %>"" costs $<%= product.price %>
      Sold by <%= product.user.name %>
    </li>
  <% end %>
</ul>

The issue is with <%= product.user.name %> , which gives the error:

NoMethodError in Products#index undefined method `name' for nil:NilClass.

My controller files are vanilla and my routes.rb has:

ControllerExercise::Application.routes.draw do
  get '/products' => 'products#index'
  resources :users
end

This is in the models:

class Product < ActiveRecord::Base
  belongs_to :user
  attr_accessible :name, :price
end

class User < ActiveRecord::Base
  has_many :products
  attr_accessible :job_title, :name
end

Any and all help is greatly appreciated.

Answer 1

你也可以试试

product.user.try(:name)

Answer 2

In your database i suspect some of your product data does not contain sold_by_id or user_id. So its getting product.user nil as no user is associated with the product. Instead of

<%= product.user.name %>

use

<%= product.user.name if product.user %>

to skip this exception.

So your index.html.erb becomes

<% lots_of_products = Product.includes(:user).all %>
<ul>
  <% lots_of_products.each do |product| %>
<li>
  Product Name: <%= product.name %> costs $<%= product.price %>
  Sold by <%= product.user.name if product.user %>
</li>
<% end %>
</ul>

Answer 3

I also came up against this and used a simple if else statement, because I still wanted to show something even if the username didn't exist

<!-- SHOW USERNAME IF IT EXISTS -->

<% if product.user %>
   <p><strong><%= product.user.name %></strong></p>
<% else %>
   <p><strong>Anonymous</strong></p>
<% end %>

Answer 4

A great workaround to this is ruby's safe access operator &.

see http://mitrev.net/ruby/2015/11/13/the-operator-in-ruby/

It was invented by Matz (ruby's founder) for this very situation. In your case it would look like

product.user&.name

It returns nil if the name attribute is not defined. Keep in mind its only available in ruby 2.3.0 and higher.