examples.rb

# Add a new child to this person
@person.children_attributes = [ { :name => 'Son' } ]
@person.children #=> [ <#Person: name: 'Son'> ]
@person.children.clear

# Add two new children to this person
@person.children_attributes =
  [ { :name => 'Son' }, { :name => 'Daughter' } ]
@person.save
@person.children #=> [ <#Person: name: 'Son'>, <#Person: name: 'Daughter'> ]

# Edit the son (assuming id == 1)
@person.children_attributes = [ { :id => 1, :name => 'Lad' } ]
@person.save
  #=> the son's name is now 'Lad'

# Edit the daughter (id == 2) and add a new offspring
@person.children_attributes =
  [ { :id => 2, :name => 'Lassie' }, { :name => 'Pat' } ]
@person.save
  #=> the daughter's name is now 'Lassie' and there's a new offspring called 'Pat'

# Remove Pat (id = 3), we don't like him/her
@person.children_attributes = [ :id => 3, '_delete' => '1' } ]
@person.save
  #=> Pat is now deleted

form.erb

<% form_for @person do |person_form| %>

  <%= person_form.label :name %>
  <%= person_form.text_field :name %>

  <% person_form.fields_for :children do |child_form| %>

    <%= child_form.label :name %>
    <%= child_form.text_field :name %>

    <% unless child_form.object.new_record? %>
      <%= child_form.check_box '_delete' %>
      <%= child_form.label '_delete', 'Remove' %>
    <% end %>

  <% end %>

  <%= submit_tag %>
<% end %>

person.rb

class Person < ActiveRecord::Base

  validates_presence_of :name

  has_many :children, :class_name => 'Person'
  accepts_nested_attributes_for :children, :allow_destroy => true
    # can also be used on has_one etc.. associations

end