Rails links not connecting properly - ruby-on-rails

Question

I am a rails beginner and I have created 3 models/controllers/views using rails generate scaffold:
Subjects, which have many topics
Topics, which have many notes
Notes
When I go to http://localhost:3000/subjects/1/topics, Rails lists the empty list of topics and when the 'New Topic' link is clicked, you are taken to http://localhost:3000/topics/new.
Should and how do I get the link for 'New Topic' to take the user to http://localhost:3000/subjects/:id/topics/new instead of http://localhost:3000/topics/new and should the new topic form submit to http://localhost:3000/subjects/:id/topics/new instead of http://localhost:3000/topics?
views/topics/index:
<h1>Listing topics</h1>
<table>
<tr>
<th>Name</th>
<th></th>
<th></th>
<th></th>
</tr>
<% #topics.each do |topic| %>
<tr>
<td><%= topic.name %></td>
<td><%= link_to 'Show', topic %></td>
<td><%= link_to 'Edit', edit_topic_path(topic) %></td>
<td><%= link_to 'Destroy', topic, method: :delete, data: { confirm: 'Are you sure?' } %></td>
</tr>
<% end %>
</table>
<br />
<%= link_to 'New Topic', new_topic_path %>
controllers/topics:
def new
#topic = Topic.new
respond_to do |format|
format.html # new.html.erb
format.json { render json: #topic }
end
end
def edit
#topic = Topic.find(params[:id])
end
def create
#topic = Topic.new(params[:topic])
#topic.subject_id = params[:project_id]
respond_to do |format|
if #topic.save
format.html { redirect_to subject_path(#topic.subject_id), notice: 'Topic was successfully created.' }
format.json { render json: #topic, status: :created, location: #topic }
else
format.html { render action: "new" }
format.json { render json: #topic.errors, status: :unprocessable_entity }
end
end
end
new topics form:
<%= form_for(#topic) do |f| %>
<% if #topic.errors.any? %>
<div id="error_explanation">
<h2><%= pluralize(#topic.errors.count, "error") %> prohibited this topic from being saved:</h2>
<ul>
<% #topic.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %>
</ul>
</div>
<% end %>
<div class="field">
<%= f.label :name %><br />
<%= f.text_field :name %>
</div>
<div class="actions">
<%= f.submit %>
</div>
<% end %>
routes:
resources :subjects do
resources :topics do
resources :notes
end
end
resources :notes
resources :topics
resources :subjects
root :to => 'subjects#index'

Answer 1

when you go to http://localhost:3000/subjects/1/topics the action index of controller TopicsController is called with the params[:subject_id] set to 1.
So in your controller action, you have to check for this parameter and filter the topics if it is given
def index
if params[:subject_id].present?
#subject=Subject.find params[:subject_id]
#topics=#subject.topics
else
#topics=Topic.all
end
end
and in your index view, you have to use this url, if #subject ist present:
<%= link_to 'New Topic', #subject.present? ? new_subject_topic_path(#subject) : new_topic_path %>
in your topics new action again you get the :subject_id as a parameter:
def new
#subject=Subject.find params[:subject_id]
#topic = #subject.topics.new
end
then in your topics form, you can forward the subject_id in a hidden field:
<%= form_for(#topic) do |f| %>
...
<%= f.hidden_field :subject_id %>
...
<% end %>
The rest of your TobicsController can stay the same. The Topic is assiciated to the Subject by the subject_id

Answer 2

Once you have declared the nested resources, don't declare those resources again.
Deep nesting of resources is not recommended. You should think about doing shallow nesting, and you can read about this in the Rails Routing Guide.
One way to avoid deep nesting (as recommended above) is to generate
the collection actions scoped under the parent, so as to get a sense
of the hierarchy, but to not nest the member actions. In other words,
to only build routes with the minimal amount of information to
uniquely identify the resource, like this:
resources :posts do
resources :comments, only: [:index, :new, :create]
end
resources :comments, only: [:show, :edit, :update, :destroy]
This idea strikes a balance between descriptive routes and deep
nesting.