# config/routes.rb

resources :groups, only: "index" do
  resources :discussions, only: %w[index show]
end

# app/controllers/groups_controller.rb
class GroupsController < ApplicationController
  def index
    @groups = Group.order(:name)
  end
end

# app/controllers/discussions_controller.rb
class DiscussionsController < ApplicationController
  before_action :load_group

  def index
    @discussions = @group.discussions.order(created_at: :desc)
  end

  def show
    @discussion = @group.discussions.find(params[:id])
    @messages = @discussion.messages.order(:created_at)
  end

private

  def load_group
    @group = Group.find(params[:group_id])
  end
end

<!-- app/views/groups/index.html.erb -->

<ul>
<% @groups.each do |group| %>
  <li><%= link_to group.name, group_discussions_path(group) %></li>
<% end %>
</ul>

<!-- app/views/discussions/index.html.erb -->

<h1><%= @group.name %></h1>

<ul>
<% @discussions.each do |discussion| %>
  <li><%= link_to discussion.subject, group_discussion_path(@group, discussion) %></li>
<% end %>
</ul>

<!-- app/views/discussions/show.html.erb -->

<h1><%= @discussion.subject %></h1>

<% @messages.each do |message| %>
  <p>From: <%= message.from.name %></p>
  <p>Timestamp: <%= message.created_at.to_fs(:timestamp) %></p>

  <%= simple_format message.content %>
<% end %>