My version of this, added a few missing things and edited for Rails 4

creating-nested-resources-in-ruby-on-rails-3-and-updating-scaffolding-links-and-redirection.markdown

In this article, I'll walk through a basic Rails (4.0.0) setup for creating a nested resource for two models. Nested resources work well when you want to build out URL structure between two related models, and still maintain a RESTful convention. This code assumes you are running RVM to manage Ruby/Gem versions, and Git for version control.

Creating a new Rails project

$ mkdir family # create rvm gemset 
$ echo "rvm use --create ruby-1.9.2@family" > family/.rvmrc 
$ cd family # install rails 
$ gem install rails # create new rails project 
$ rails new . # version control 
$ git init 
$ git add . 
$ git commit -am "new rails project"

Create two models (Parent & Child)

# Parent model 
$ rails generate scaffold Parent name:string 
$ git add . 
$ git commit -am "rails generate scaffold Parent name:string" 
# Child model 
$ rails generate scaffold Child name:string parent_id:integer 
$ git add . 
$ git commit -am "rails generate scaffold Child name:string parent_id:integer" 
# Create db (defaults to SQLite3) 
$ rake db:migrate 
# version control 
$ git add db/schema.rb 
$ git commit db/schema.rb -m "created database schema"

Review un-nested routes

$ rake routes
   children GET    /children(.:format)          children#index
            POST   /children(.:format)          children#create
  new_child GET    /children/new(.:format)      children#new
 edit_child GET    /children/:id/edit(.:format) children#edit
      child GET    /children/:id(.:format)      children#show
            PUT    /children/:id(.:format)      children#update
            DELETE /children/:id(.:format)      children#destroy
    parents GET    /parents(.:format)           parents#index
            POST   /parents(.:format)           parents#create
 new_parent GET    /parents/new(.:format)       parents#new
edit_parent GET    /parents/:id/edit(.:format)  parents#edit
     parent GET    /parents/:id(.:format)       parents#show
            PUT    /parents/:id(.:format)       parents#update
            DELETE /parents/:id(.:format)       parents#destroy

Adding model relationships

# file: app/models/parent.rb
class Parent < ActiveRecord::Base
  attr_accessible :name
  has_many :children
end

# file: app/models/child.rb
class Child < ActiveRecord::Base
  attr_accessible :name, :parent_id
  belongs_to :parent
end

# version control
$ git commit app/models -m "added relationships to models"

Nesting the routes

# file: config/routes.rb
- resources :parents
- resources :children

+ resources :parents do
+    resources :children
+ end

$ rake routes
  parent_children GET    /parents/:parent_id/children(.:format)          children#index
                  POST   /parents/:parent_id/children(.:format)          children#create
 new_parent_child GET    /parents/:parent_id/children/new(.:format)      children#new
edit_parent_child GET    /parents/:parent_id/children/:id/edit(.:format) children#edit
     parent_child GET    /parents/:parent_id/children/:id(.:format)      children#show
                  PUT    /parents/:parent_id/children/:id(.:format)      children#update
                  DELETE /parents/:parent_id/children/:id(.:format)      children#destroy
          parents GET    /parents(.:format)                              parents#index
                  POST   /parents(.:format)                              parents#create
       new_parent GET    /parents/new(.:format)                          parents#new
      edit_parent GET    /parents/:id/edit(.:format)                     parents#edit
           parent GET    /parents/:id(.:format)                          parents#show
                  PUT    /parents/:id(.:format)                          parents#update
                  DELETE /parents/:id(.:format)                          parents#destroy

Adding test data via Rails console

$ rails c

> dad = Parent.new(:name => 'Paul')
 => #<Parent id: nil, name: "Paul", created_at: nil, updated_at: nil> 

> dad.save
   (0.1ms)  begin transaction
  SQL (20.0ms)  INSERT INTO "parents" ("created_at", "name", "updated_at") VALUES (?, ?, ?)  [["created_at", Fri, 06 Apr 2012 16:13:17 UTC +00:00], ["name", "Paul"], ["updated_at", Fri, 06 Apr 2012 16:13:17 UTC +00:00]]
   (2.4ms)  commit transaction
 => true 

> son = dad.children.new(:name => 'Eric')
 => #<Child id: nil, name: "Eric", parent_id: 1, created_at: nil, updated_at: nil> 

> daughter = dad.children.new(:name => 'Mara')
 => #<Child id: nil, name: "Mara", parent_id: 1, created_at: nil, updated_at: nil> 

> dad.save
 => true
> exit

Adding a private controller method to load the Parent object for each method

# file: app/controllers/children_controller.rb
@@ -1,4 +1,7 @@
 class ChildrenController < ApplicationController
+
+  before_filter :load_parent
+
   # GET /children
   # GET /children.json
   def index
@@ -80,4 +83,11 @@ class ChildrenController < ApplicationController
       format.json { head :no_content }
     end
   end
+
+  private
+
+    def load_parent
+      @parent = Parent.find(params[:parent_id])
+    end
+
 end

At this point, each controller and view for the Child class model needs to be adjusted (links, redirection, form, etc)

Method: children#index

# file: app/controllers/children_controller.rb

   def index
-    @children = Child.all
+    @children = @parent.children.all

# file: app/views/children/index.html.erb

-    <td><%= link_to 'Show', child %></td>
-    <td><%= link_to 'Edit', edit_child_path(child) %></td>
-    <td><%= link_to 'Destroy', child, confirm: 'Are you sure?', method: :delete %></td>
+    <td><%= link_to 'Show', parent_child_path(@parent, child) %></td>
+    <td><%= link_to 'Edit', edit_parent_child_path(@parent, child) %></td>
+    <td><%= link_to 'Destroy', [@parent, child], confirm: 'Are you sure?', method: :delete %></td>
 
-<%= link_to 'New Child', new_child_path %>
+<%= link_to 'New Child', new_parent_child_path(@parent) %>

Method: children#new

# file: app/controllers/children_controller.rb

   def new
-    @child = Child.new
+    @child = @parent.children.new

# file: app/views/children/_form.html.erb

-<%= form_for(@child) do |f| %>
+<%= form_for([@parent, @child]) do |f| %>

# file: app/views/children/new.html.erb

-<%= link_to 'Back', children_path %>
+<%= link_to 'Back', parent_children_path(@parent) %>

Method: children#create

# file: app/controllers/children_controller.rb

   def create
-    @child = Child.new(params[:child])
+    @child = @parent.children.new(params[:child])
 
     respond_to do |format|
       if @child.save
-        format.html { redirect_to @child, notice: 'Child was successfully created.' }
+        format.html { redirect_to [@parent, @child], notice: 'Child was successfully created.' }

Method: children#show

# file: app/controllers/children_controller.rb

   def show
+    @child = @parent.children.find(params[:id])

# file: app/views/children/show.html.erb

-<%= link_to 'Edit', edit_child_path(@child) %> |
-<%= link_to 'Back', children_path %>
+<%= link_to 'Edit', edit_parent_child_path(@parent, @child) %> |
+<%= link_to 'Back', parent_children_path(@parent) %>

Method: children#edit

# file: app/controllers/children_controller.rb

   def edit
-    @child = Child.find(params[:id])
+    @child = @parent.children.find(params[:id])

# file: app/views/children/edit.html.erb

-<%= link_to 'Show', @child %> |
-<%= link_to 'Back', children_path %>
+<%= link_to 'Show', parent_child_path(@parent, @child) %> |
+<%= link_to 'Back', parent_children_path(@parent) %>

Method: children#update

# file: app/controllers/children_controller.rb

   def update
+    @child = @parent.children.find(params[:id])
 
     respond_to do |format|
       if @child.update_attributes(params[:child])
-        format.html { redirect_to @child, notice: 'Child was successfully updated.' }
+        format.html { redirect_to [@parent, @child], notice: 'Child was successfully updated.' }

Method: children#destroy

# file: app/controllers/children_controller.rb

   def destroy
+    @child = @parent.children.find(params[:id])
     @child.destroy
 
     respond_to do |format|
-      format.html { redirect_to children_url }
+      format.html { redirect_to parent_children_path(@parent) }

At this point, the default scaffolding's links and redirection have been updated to work with the nested routes.

This seems to be the only fork that mentions the changes in routes.rb. Thanks!