Passing validation errors array from one controller to another - ruby-on-rails

Question

I have Comment belongs_to Post and Post has_many Comments, the comment model as following:
class Comment < ApplicationRecord
belongs_to :post
belongs_to :user
validates :text, presence: true
end
The form which adds new comments is located in Posts show view, as following:
<%= form_with(model: [ #post, #post.comments.build ], local: true) do |form| %>
<% if #comment.errors.any?%>
<div id="error_explanation">
<ul>
<% #comment.errors.messages.values.each do |msg| %>
<%msg.each do |m| %>
<li><%= m %></li>
<%end %>
<% end %>
</ul>
</div>
<% end %>
<p>
<%= form.text_area :text , {placeholder: true}%>
</p>
<p>
<%= form.submit %>
</p>
<% end %>
The Comments create action, as following:
class CommentsController < ApplicationController
def create
#post = Post.find(params[:post_id])
#comment = Comment.new(comment_params)
#comment.post_id = params[:post_id]
#comment.user_id = current_user.id
if #comment.save
redirect_to post_path(#post)
else
render 'posts/show'
end
end
private
def comment_params
params.require(:comment).permit(:text)
end
end
I need to render the posts/show page to show Comment validation errors, but the problem is I'm in CommentsController controller not PostsController so all objects used in pages/show view will be null.
How can I pass the #comment object to pages/show?
I thought about using flash array, but I'm looking for more conventional way.

Answer 1

Yep, rendering pages/show will just use the view template, but not the action, so anything you define in the pages#show controller action won't be available. #comment will be available though as you define that in the comments#create action. Without seeing the PostsController I don't know what else you're loading in the pages#show action - you could consider moving anything required in both to a method on ApplicationController, then calling from both places. Another option would be to change your commenting process to work via AJAX (remote: true instead of local: true on the form), and responding with JS designed to re-render just the comment form (you can move it into a partial used both in pages/show.html.erb and the comments#create response).
Couple of other notes on your code above - in comments#create, you can use:
#comment = #post.comments.new(comment_params)
to avoid needing to set the post_id on #comment manually.
For the form, I'd be tempted to setup a new comment in pages#show:
#comment = #post.comments.build
And then reference that in the form, it'll make it easier if you do re-use that between pages#show and comments#create:
<%= form_with(model: [ #post, #comment ], local: true) do |form| %>
Hope that helps!