Rails Nested Forms Example - Formtastic, Cocoon. Tutorial is here - http://maxivak.com/rails-3-nested-models-in-one-form-using-formtastic-and-cocoon-gems/

db_script_mysql

CREATE TABLE IF NOT EXISTS `projects` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci ;

CREATE TABLE IF NOT EXISTS `tasks` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `project_id` int(11) NOT NULL,
  `title` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci ;

form

# app/views/projects/_form.html.haml

= semantic_form_for @project  do |f|
  - if @project.errors.any?
    #form_errors.error
      = f.semantic_errors :state
      %h2
        #{pluralize(@item.errors.count, "error")}:
      %ul
        - @project.errors.full_messages.each do |msg|
          %li
            = msg


  = f.inputs :name => "Basic" do
    = f.input :title
    = f.input :description, :required => false, :input_html => {  :style => "width:500px; height:120px;" }, :rows=>4
     
     
  = f.inputs :name => "Tasks" do
    = f.semantic_fields_for :tasks do |task|
      = render "task_fields", :f => task

    .links
      = link_to_add_association 'Add task', f, :tasks

     
   .clear
    %br
    %br

  .form-group
    = f.actions do
      = f.action :submit, :as => :button
      = f.action :cancel, :as => :link, :label => "Cancel"

js

# app/assets/javascripts/application.js

//= require cocoon

models

# app/models/project.rb
class Project < ActiveRecord::Base
  attr_accessible :title, :description, :tasks_attributes

  has_many :tasks, :dependent => :destroy
  accepts_nested_attributes_for :tasks, :reject_if => lambda { |a| a[:title].blank? }, :allow_destroy => true

..

end


# app/models/task.rb
class Task < ActiveRecord::Base
  attr_accessible :project_id, :title

  belongs_to :project

  validates_presence_of :title

  
..
end

task_fields

# app/views/project/_task_fields.html.haml

.nested-fields
  = f.inputs do
    = f.input :title
    = link_to_remove_association "Remove", f