Added Tag as a quick test of has_and_belongs_to_many with nested_params.

app/controllers/projects_controller.rb

@@ -10,6 +10,7 @@ def show
   def new
     @project = Project.new
     @project.tasks.build
+    @project.tags.build
   end
 
   def create
@@ -26,6 +27,7 @@ def edit
     @project = Project.find(params[:id])
     # add an extra new record for debugging purposes
     @project.tasks.build
+    @project.tags.build
   end
 
   def update

app/helpers/projects_helper.rb

@@ -8,4 +8,14 @@ def add_task_link(name, form)
       }
     end
   end
+
+  def add_tag_link(name, form)
+    link_to_function name do |page|
+      tag = render(:partial => 'tag', :locals => { :pf => form, :tag => Tag.new })
+      page << %{
+        var new_tag_id = "new_" + new Date().getTime();
+        $('tags').insert({ bottom: "#{ escape_javascript tag }".replace(/new_\\d+/g, new_tag_id) });
+      }
+    end
+  end
 end

app/models/project.rb

@@ -2,6 +2,7 @@ class Project < ActiveRecord::Base
   # Automatically turns on autosave and thus also validates
   has_one :author, :nested_params => true
   has_many :tasks, :dependent => :destroy, :nested_params => true, :destroy_missing => true
+  has_and_belongs_to_many :tags, :nested_params => true, :destroy_missing => true
 
   validates_presence_of :name
 end

app/models/tag.rb

@@ -0,0 +1,3 @@
+class Tag < ActiveRecord::Base
+  validates_presence_of :name, :message => "can't just be blank"
+end

app/views/projects/_form.html.erb

@@ -32,6 +32,24 @@
   <p>
     <%= add_task_link "Add a task", f %>
   </p>
+
+  <div id="tags">
+    <%#= render :partial => 'task', :collection => @project.tasks, :locals => { :pf => f } %>
+    <!-- For the sake of demoing the fields_for which yields each record. -->
+    <% f.fields_for :tags do |tf| %>
+      <div class="tag">
+        <p>
+          <%= tf.label :name, "Tag:" %>
+          <%= tf.text_field :name %>
+          <%= link_to_function "remove", "$(this).up('.tag').remove()" %>
+        </p>
+      </div>
+    <% end %>
+  </div>
+  <p>
+    <%= add_tag_link "Add a tag", f %>
+  </p>
+
   <p>
     <%= f.submit "Submit" %>
   </p>

app/views/projects/_tag.html.erb

@@ -0,0 +1,9 @@
+<div class="tag">
+<% pf.fields_for :tags, tag do |f| %>
+  <p>
+    <%= f.label :name, "Tag:" %>
+    <%= f.text_field :name %>
+    <%= link_to_function "remove", "$(this).up('.tag').remove()" %>
+  </p>
+<% end %>
+</div>

app/views/projects/show.html.erb

@@ -13,11 +13,19 @@
 <h3>Tasks</h3>
 
 <ul>
-  <% for task in @project.tasks %>
+  <% @project.tasks.each do |task| %>
     <li><%=h task.name %></li>
   <% end %>
 </ul>
 
+<h3>Tags</h3>
+
+<ul>
+  <% @project.tags.each do |tag| %>
+    <li><%=h tag.name %></li>
+  <% end %>
+</ul>
+
 <p>
   <%= link_to "Edit", edit_project_path(@project) %> |
   <%= link_to "Destroy", @project, :confirm => 'Are you sure?', :method => :delete %> |

db/migrate/20081009223323_create_tags.rb

@@ -0,0 +1,12 @@
+class CreateTags < ActiveRecord::Migration
+  def self.up
+    create_table :tags do |t|
+      t.string :name
+      t.timestamps
+    end
+  end
+
+  def self.down
+    drop_table :tags
+  end
+end

db/migrate/20081009223404_create_projects_tags_join_table.rb

@@ -0,0 +1,14 @@
+class CreateProjectsTagsJoinTable < ActiveRecord::Migration
+  def self.up
+    create_table :projects_tags, :id => false do |t|
+      t.integer :project_id
+      t.integer :tag_id
+    end
+    add_index :projects_tags, :project_id
+    add_index :projects_tags, :tag_id
+  end
+
+  def self.down
+    drop_table :projects_tags
+  end
+end

db/schema.rb

@@ -9,7 +9,7 @@
 #
 # It's strongly recommended to check this file into your version control system.
 
-ActiveRecord::Schema.define(:version => 20080918210237) do
+ActiveRecord::Schema.define(:version => 20081009223404) do
 
   create_table "authors", :force => true do |t|
     t.integer  "project_id"
@@ -26,6 +26,20 @@
     t.datetime "updated_at"
   end
 
+  create_table "projects_tags", :id => false, :force => true do |t|
+    t.integer "project_id"
+    t.integer "tag_id"
+  end
+
+  add_index "projects_tags", ["tag_id"], :name => "index_projects_tags_on_tag_id"
+  add_index "projects_tags", ["project_id"], :name => "index_projects_tags_on_project_id"
+
+  create_table "tags", :force => true do |t|
+    t.string   "name"
+    t.datetime "created_at"
+    t.datetime "updated_at"
+  end
+
   create_table "tasks", :force => true do |t|
     t.integer  "project_id"
     t.string   "name"

test/fixtures/tags.yml

@@ -0,0 +1,7 @@
+# Read about fixtures at http://ar.rubyonrails.org/classes/Fixtures.html
+
+one:
+  name: MyString
+
+two:
+  name: MyString

test/unit/tag_test.rb

@@ -0,0 +1,8 @@
+require 'test_helper'
+
+class TagTest < ActiveSupport::TestCase
+  # Replace this with your real tests.
+  test "the truth" do
+    assert true
+  end
+end