Empty Post in Rails 5.2 - ruby-on-rails

Question

I'm trying to add the helper _form of my Comment model inside Advertisement view. I call the helper <%= render 'comments/form', comment: #comment %>in the route /advertisement/:id
So here is my comments _form :
<%= form_with(model: comment, local: true, url: "/comments") do |form| %>
<% if comment && comment.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(comment.errors.count, "error") %> prohibited this comment from being saved:</h2>
<ul>
<% comment.errors.full_messages.each do |message| %>
<li><%= message %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="form-group">
<%= form.label :content %>
<%= form.text_area :content, class:"form-field" %>
</div>
<%= form.hidden_field :advertisment_id, value:params[:id] %>
<div class="actions">
<%= form.submit "Envoyer", class:"btn btn-primary", url: 'comments' %>
</div>
<% end %>
I specify the url: "/comments" because I'm at /advertisment/:id and by default the action is aimed to here.
But when the post reached my comment controller, it can't read the params.
ActionController::ParameterMissing in CommentsController#create
The strange part is that I've access to the params :
{"utf8"=>"✓", "authenticity_token"=>"0+swOqHPEHN2Gwh0TO3iC7VPRz4ROLoBlkaMkOdnjjYxWHoDer7AwrgnQpu+9VHfSY90yMSRsNp8ojvPJxuzmQ==", "content"=>"Test", "advertisment_id"=>"1", "commit"=>"Envoyer"}
So here is the comment controller :
class CommentsController < ApplicationController
before_action :set_comment, only: [:show, :edit, :update, :destroy]
# [...]
# POST /comments
# POST /comments.json
def create
#comment = Comment.new(comment_params.merge(:user_id => #session_user.id))
respond_to do |format|
if #comment.save
format.html { redirect_to #comment, notice: 'Comment was successfully created.' }
format.json { render :show, status: :created, location: #comment }
else
format.html { render :new }
format.json { render json: #comment.errors, status: :unprocessable_entity }
end
end
end
# [...]
private
# Use callbacks to share common setup or constraints between actions.
def set_comment
#comment = Comment.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def comment_params
params.require(:comment).permit(:content, :advertisment_id)
end
end
Thanks a lot for your help

Answer 1

It probably because your #comment is nil so that form_with works like the old form_tag helper.
By using form_with, a field like <%= form.text_field :content %>
If your model is presented, it will generate <input type="text" name="comment[content]" />
If your model is not presented, it will generate <input type="text" name="content" />
In the second case, your strong params check will reject the naked params
To be able to solve it, simply assign a valid model to your render
<%= render 'comments/form', comment: Comment.new %>

Answer 2

According to the docs here, if you specific url option in form_with without the scope option, the params received will be missing prefixes in there name. Therefore
params.require(:comment).permit(:content, :advertisment_id)
will raise an error since the params does not include comment.
You should remove url option in your form_with or use url and scope options at the same time.