passing params throught rails forms - html

Question

I am using rails 4 and have a subject and comment models. Subject is a one to many relationship with comments. I want a simple page that can add comments to many subjects on the same page. So in my form I know how to submit a comment to create but I dont know how to find the right subject in my controller to add it to. Any advice?
class CommentsController < ApplicationController
def create
comment = Comment.create(comment_params)
if comment.save
# The line below is incorrect, I dont know what to do
Subject.find(params[:subject_id]).comments << comment
redirect_to(:controller => 'static_pages', action: 'home')
end
end
def new
end
private
def comment_params
params.require(:comment).permit(:text, :user_name)
end
end
StaticPages#home Find me in
app/views/static_pages/home.html.erb
<% #subjects.each do |subject| %>
<div class="subjects <%= cycle('odd', 'even') %>">
<h1><%= subject.name %></h1>
<h3><%= subject.description %></h3>
<% subject.comments.each do |comment|%>
<div class="comment">
<h4><%= comment.user_name%></h4>
<%= comment.text %>
</div>
<% end %>
<%= form_for(#comment) do |f| %>
<%= f.label :user_name %>
<%= f.text_field :user_name %>
<%= f.label :text %>
<%= f.text_field :text %>
<%= f.submit('Create comment', subject_id: subject.id) %>
<% end %>
</div>
<% end %>

Answer 1

The simplest way would be to populate the subject_id attribute of your #comment form, like this:
<%= form_for(#comment) do |f| %>
<%= f.label :user_name %>
<%= f.text_field :user_name %>
<%= f.label :text %>
<%= f.text_field :text %>
<%= f.hidden_field :subject_id, value: subject.id %>
<%= f.submit('Create comment', subject_id: subject.id) %>
<% end %>
This will populate the subject_id attribute of your new Comment object, which will essentially associate it through Rails' backend:
#app/controllers/your_controller.rb
Class YourController < ApplicationController
def create
#comment = Comment.new comment_params
#comment.save
end
private
def comment_params
params.require(:comment).permit(:subject_id, :text, :user_name)
end
end
--
foreign_keys
This works because of the Rails / relational database foreign_keys structure
Every time you associate two objects with Rails, or another relational database system, you basically have a database column which links the two. This is called a foreign_key, and in your case, every Comment will have the subject_id foreign_key column, associating it with the relevant subject
So you may have many different forms using the same #comment variable - the trick is to populate the foreign_key for each one