Showing associated model errors in Rails 6 form

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I'm using Rails 6.0.3.4 and Ruby 2.7.2. Using the Rails getting started tutorial as an example, I'm wondering how to show form validation errors of an associated model.

Show page

<p>
  <strong>Title:</strong>
  <%= @article.title %>
</p>
 
<p>
  <strong>Text:</strong>
  <%= @article.text %>
</p>
 
<h2>Comments</h2>
<%= render @article.comments %>
 
<h2>Add a comment:</h2>
<%= render 'comments/form' %>
 
<%= link_to 'Edit', edit_article_path(@article) %> |
<%= link_to 'Back', articles_path %>

Comments form (this is the form in question)

<%= form_with(model: [ @article, @article.comments.build ], local: true) do |form| %>
  <p>
    <%= form.label :commenter %><br>
    <%= form.text_field :commenter %>
  </p>
  <p>
    <%= form.label :body %><br>
    <%= form.text_area :body %>
  </p>
  <p>
    <%= form.submit %>
  </p>
<% end %>

Models

class Comment < ApplicationRecord
  belongs_to :article

  validates :commenter, presence: true
end

class Article < ApplicationRecord
  has_many :comments, dependent: :destroy
  validates :title, presence: true,
                    length: { minimum: 5 }
end

For a single model articles form, errors could be shown like this.

<% if @article.errors.any? %>
    <div id="error_explanation">
        <h2>
        <%= pluralize(@article.errors.count, "error") %> prohibited
        this article from being saved:
        </h2>
        <ul>
        <% @article.errors.full_messages.each do |msg| %>
            <li><%= msg %></li>
        <% end %>
        </ul>
    </div>
<% end %>

How do I show errors for the comments form? When I submit a comment without a commentor, it does not save, so validations are happening, but I'm not sure how to show errors for this type of form.

<% if @???????.errors.any? %> ###### What do I say here to get the comment errors?
    <div id="error_explanation">
        <h2>
        <%= pluralize(@?????.errors.count, "error") %> prohibited
        this article from being saved:
        </h2>
        <ul>
        <% @?????.errors.full_messages.each do |msg| %>
            <li><%= msg %></li>
        <% end %>
        </ul>
    </div>
<% end %>
ruby-on-rails ruby
1 Answers

1. Create a reusable partial for errors

# app/views/shared/_errors.html.erb
<div class="error_explanation">
  <h2><%= pluralize(object.errors.count, "error") %> prohibited
 this <%= object.model_name.singular %> from being saved:</h2>
  <ul>
  <% object.errors.full_messages.each do |msg| %>
    <li><%= msg %></li>
  <% end %>
  </ul>
</div>

And a little helper method:

# app/helpers/application_helper.rb
module ApplicationHelper
  # Displays the errors for a model instance if there are any
  def display_errors_for(object)
    return unless object.errors.any?
    render partial: 'shared/errors', 
           locals: { object: object }
  end
end

2. Get the object from the form builder

You can always access the model wrapped by the form builder instance through the #object method instead of using a instance variable:

<%= form_with(model: [ @article, @comment ], local: true) do |form| %>
  # ...
  <%= display_errors_for(form.object) %>
<% end %>

And like magic you can add errors to any form with a single line.

Do not use @article.comments.build. That will always bind the form to a new instance of comment instead of displaying the errors! It will also remove anything the user entered into the form... Assign the variable in the controller. I have no idea how this snuck its way into the guides.

class ArticlesController < ApplicationController
  def show
    @article = Article.find(params[:id])
    @comment = @article.comments.new
  end
end
user_5448250
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