Showing form error messages - ruby-on-rails

Question

I'm having trouble getting my redirect and error messages to work. From what I've read you cant get a forms errors to show up when you use redirect so I am trying to use render after it fails.
I have a new post form on a topic page. The url is "topic/1". If you make a post about the topic and something is wrong with the input I want it to go back to the page at topic/1 and display errors and I cant figure out how to get it to go back. Redirect (:back) does what I want but doesnt show the forms errors.
The form on the topic's show.html page:
<%= form_for(#post) do |f| %>
<%= render 'shared/post_error_messages' %>
<%= f.label :title, "Post Title" %>
<%= f.text_field :title %>
<%= f.label :content %>
<%= f.text_field :content %>
<%= f.hidden_field :parent_id, value: 0 %>
<%= f.hidden_field :topic_id, value: #topic.id %>
<%= f.hidden_field :user_id, value: current_user.id %>
<%= f.submit "Create Post" , class: "btn btn-small btn-primary" %>
<% end %>
Create action in the Posts controller
def create
#post = Post.new(post_params)
#topic = Topic.find_by(id: params[:topic_id])
if #post.save
redirect_to #post
else
#topic = Topic.new
render "/topics/show"
end
end
I guess I'm mostly trying to do the render with the id from the page that the form was originally on.

Answer 1

Errors
The problem isn't anything to do with the way you're rendering the form (render or redirect) - it's to do with the way you're handling your ActiveRecord object.
When you use form_for, Rails will append any errors into the #active_record_object.errors method. This will allow you to call the following:
form_for error messages in Ruby on Rails
<%= form_for #object do |f| %>
<% #location.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
<% end %>
This only works if you correctly create your ActiveRecord object, which you seem to do
--
Nested
#config/routes.rb
resources :topics do
resources :posts, path: "", path_names: {new: ""}, except: [:index] #-> domain.com/topics/1
end
You'll be much better using the following setup for a nested route:
<%= form_for [#topic, #post] do |f| %>
...
<% end %>
This allows you to create a form which will route to the topics_posts_path, which is basically what you need. The controller will then balance that by using the following:
#app/controllers/topics_controller.rb
Class TopicsController < ApplicationController
def new
#topic = Topic.find params[:topic_id]
#post = Post.new
end
def create
#topic = Topic.find params[:topic_id]
#post = Post.new post_params
end
private
def post_params
params.require(:post).permit(:attributes)
end
end

Answer 2

You are overwriting the Topic you original found with a brand new, empty one - which shouldn't be necessary and which is causing the posts related to it to disappear.
Also - if your topic and post are related - you should create the post on the appropriate association #topic.posts instead of the main Post class.
The #topic.posts.new means that the post's topic-id is automatically updated with the value of the #topic.id ... which means you don't need to set it in the hidden-field on the form.
In fact it's better if you don't - just delete that hidden field entirely.
If you add that to the first time you get a new post too (eg in topics/show) then you won't need to pass in a value to the hidden-field.
Also I'd do the same for all the other hidden-fields on the server-side too. You don't really want the user to use firebug to hack the form and add some other user's id... so do it in the create action and don't bother with the hidden field
This should work:
def create
#topic = Topic.find_by(id: params[:topic_id])
#post = #topic.posts.new(post_params)
#post.user = current_user
#post.parent_id = 0
if #post.save
redirect_to #post
else
render "/topics/show"
end
end
if it doesn't - let me know what error messages you get (so we can debug)