Why is this passing of params into link_to not working?

Question

Still can't get this working after trying several things noted in the initial comments - I've revised below to reflect those efforts / clarify a few things.

Problem summary: I cannot create a new Post (an answer) where the parent_id of that Post equals the id of the Post for the associated question. The user starts on the posts#show page for the question Post, and clicks "Answer". I'd have thought the code below would pass the id of the question Post to the answer Posts' parent_id. But it does not. It returns parent_id = nil

I have a Posts model. A Post has a post_type_id of 1 (question) or 2 (answer).

All Posts where post_type_id = 2 have a parent_id that points to the id of the question being answered.

On posts#show I have:

<% case post_type %> 
  <% when 1 %>
<%= link_to 'Answer', new_post_path(:parent_id => @post.id) %>
  <% else %>
<% end %>

When I click on that it takes me to the new_post page and the url is: //localhost:3000/posts/new?parent_id=6

(The question in this example has id = 6, so I think that looks correct.)

In my Posts Controller I have:

  def new
    @post = Post.new
  end

  def show
  end

  def create
    @post = Post.new(post_params)
    @post.user_id = current_user.id
    @post.parent_id = params[:parent_id]

    respond_to do |format|
      if @post.save
        format.html { redirect_to @post, notice: 'Post was successfully created.' }
        format.json { render :show, status: :created, location: @post }
      else
        format.html { render :new }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
    end

  def post_params
    params.require(:post).permit(:user_id, :post_type_id, :title, :content, :accepted_answer_id, :parent_id, :score, :answer_count, :favorite_count)
  end

My routes are:

          posts GET    /posts(.:format)                   posts#index
                POST   /posts(.:format)                   posts#create
       new_post GET    /posts/new(.:format)               posts#new
      edit_post GET    /posts/:id/edit(.:format)          posts#edit
           post GET    /posts/:id(.:format)               posts#show
                PATCH  /posts/:id(.:format)               posts#update
                PUT    /posts/:id(.:format)               posts#update
                DELETE /posts/:id(.:format)               posts#destroy

In new.html.erb I have:

<h3>New Post</h3>
<%= render 'form' %>
<%= link_to 'Back', posts_path %>

And _form.html.erb is:

<%= simple_form_for(@post) do |f| %>
<%= f.error_notification %>

<div class="form-inputs">
<%- f.association :user %>
<%= f.input :post_type_id, collection: post_types, label: 'What do you want to do?' %>
<%= f.input :title %>
<%= f.input :content %>
<%- f.input :accepted_answer_id %>
<%- f.input :parent_id %>
<%- f.input :score %>
<%- f.input :answer_count %>
<%- f.input :favorite_count %>
</div>

<div class="form-actions">
<%= f.button :submit %>
</div>
<% end %>

I'd really appreciate any suggestions on how I can get this working.

Thanks!

Answer 1

Okay, so your user clicks on answer, it then forwards the question ID as a ROOT param (params[:parent_id])

Now, you want to tell your new action that the post you're building is an answer to a question, so this is where you should use the param

controllers/posts_controller.rb

def new
  @post = Post.new
  @post.parent_id = params[:parent_id] if params[:parent_id]

Then in your form, you can add a hidden field (User don't care about the id). I believe if the field .parent_id wasn't set, it will leave a blank value and it's fine

views/posts/_form.html.erb

<%= f.hidden_field :parent_id %>

Then when you POST your new answer, the :parent_id will be sent and you don't need to worry about it

controllers/posts_controller.rb

def create
    @post = Post.new(post_params)
    @post.user_id = current_user.id
    # No need for the below line, :parent_id is already in post_params
    # @post.parent_id = params[:parent_id]

    respond_to do |format|
      if @post.save
        format.html { redirect_to @post, notice: 'Post was successfully created.' }
        format.json { render :show, status: :created, location: @post }
      else
        format.html { render :new }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

  def post_params
    params.require(:post).permit(..., :parent_id, ...)
  end

EDIT : Answers to your questions in comments

Why does @post.parent_id = params[:parent_id] belongs to #new

With this line, you actually set the :parent_id memory variable of the "template @post object" you are preparing for the #new action. Now when you render the view, by default every form helper will LOAD the values in memory if they exist. When you're doing f.input :content, the form_builder will generate the HTML tags, but also look in the memory for @post.content. Because the model was generated blank, there is no value so it will show an empty field. So when you're calling f.hidden_field :parent_id, it will actually find the variable (thanks to the code we just added) and use it.

<%- instead of <%=

I personally never use <%-, and I had to google it, and found this question. <%- will just avoid line breaks, so it would work as well

Why a hidden field ?

It's just a matter of user experience. A hidden field will write in the HTML, but will hide it to the user. The user doesn't give a damn shit about your :parent_id. You are using ActiveRecord, so IDs look very nice, but if you try other databases system, your ID might look like 0bef0600008de. And your user definitely doesn't care that he's adding an answer to a question 0bef0600008de.

However h might be interested in this :

_form.html.erb

<% if @post.parent_id %> <!-- if variable parent_id exists <=> we are writing an answer -->
  <h2>You are writing an answer for the question <%= Post.find(@post.parent_id).title %></h2>
<% end %>