# 1. Form helper is used to generate form in view
<% form_tag({:action => 'update', :id => @user}) do %>
  <%= text_field 'user', 'email'  %>
<% end %>

# The generated form HTML
<form action="/admin/users/update/1" method="post">
  <input id="user_email" name="user[email]" />
</form>

# The submitted form data is in the params hash
params = {:user => {:email => "joe@user.com"}}

# The form data is passed to an ActiveRecord model
@user = User.find(params[:id])
@user.update_attributes(params[:user])

<% form_for(@post) do |f| %>
  <%= f.error_messages %>
  <%= f.label :header %>
  <%= f.text_field :header %><br />
  <%= f.label :body %>
  <%= f.text_area :body %><br />
  <%= f.submit "Create" %>
<% end %>

<% form_for :user, @admin_user,
  :url => {:action => 'save'},
  :html => {:class => 'admin_form', :multipart => true} do |f| -%>

  <%= f.error_messages %>
  <%= f.text_field :email %><br/>
  <%= text_area :user, 'bio' %><br/>
  <%= submit_tag 'Sign up' %><br/>
<% end -%>

<% form_for @person, :url => { :action => "update" } do |person_form| %>
  First name: <%= person_form.text_field :first_name %>
  Last name : <%= person_form.text_field :last_name %>

  <% fields_for @person.permission do |permission_fields| %>
    Admin?  : <%= permission_fields.check_box :admin %>
  <% end %>
<% end %>

def create
  @company = Company.new(params[:company])
  @product = Product.new(params[:product])

  Company.transaction do
    @company.save!
    @product.save!
    redirect_to :action => :show, :id => @company
  end
rescue ActiveRecord::RecordInvalid => e
  @product.valid? # Force validation as it may not have been validated
  render :action => :new
end

class Person < ActiveRecord::Base
  has_many :children, :class_name => 'Person'
  accepts_nested_attributes_for :children # Available as of Rails 2.3
end
<% form_for @person do |person_form| %>
  <% person_form.fields_for :children do |child_form| %>    
    <%= child_form.text_field :name %>
    <% unless child_form.object.new_record? %>
      <%= child_form.checkbox '_delete' %>
      <%= child_form.label '_delete', 'Remove' %>
    <% end %>
  <% end %>
  <%= submit_tag %>
<% end %>

fields_for :permission, @person.permission do |fields| ...
text_field("post", "title", "size" => 20)
password_field
hidden_field
file_field
text_area("post", "body", "cols" => 20, "rows" => 40)
check_box("post", "validated") # => 0/1 booleans
radio_button("post", "category", "rails")

  select_tag(:gender, options_for_select(['Male', 'Female']))
  select_tag(:gender, options_for_select([['Male', 'm'], ['Female', 'f']]))
  f.select(:person_id, Person.all.map {|p| [ p.name, p.id ] })
  f.collection_select(:person_id, Person.all, :id, :name)
  f.time_zone_select(:time_zone, TimeZone.us_zones, :default => "Pacific Time (US & Canada)")

distance_of_time_in_words(from_time, to_time)
time_ago_in_words(from_time)
f.date_select(:written_on,
  :start_year => 1995, :use_month_numbers => true,
  :discard_day => true, :include_blank => true)
f.datetime_select(:written_on)

class LabelFormBuilder < ActionView::Helpers::FormBuilder
  helpers = field_helpers +
            %w{date_select datetime_select time_select} +
            %w{collection_select select country_select time_zone_select} -
            %w{hidden_field label fields_for} # Don't decorate these

  helpers.each do |name|
    define_method(name) do |field, *args|
      options = args.last.is_a?(Hash) ? args.pop : {}
      label = label(field, options[:label], :class => options[:label_clas])
      @template.content_tag(:p, label +'<br/>' + super)  #wrap with a paragraph 
    end
  end
end

ActionView::Base.default_form_builder = LabelFormBuilder

# Labels and line breaks and paragraphs are generated automatically
<% form_for(@user, :builder => LabelFormBuilder) do |f| %>
  <%= f.error_messages %>
  <%= f.text_field :name %>
  <%= f.text_field :address %>
  <%= f.text_area :comment %>
  <%= f.check_box :check %>
  <%= f.submit "Update" %>
<% end %>

class User < ActiveRecord::Base
  has_attached_file :avatar, 
                    :styles => { :medium => "300x300>",
                                 :thumb => "100x100>" }
end

<% form_for :user, :html => { :multipart => true } do |form| %>
  <%= form.file_field :avatar %>
<% end %>

# Style #errorExplanation and .fieldWithErrors in your CSS file

# Default markup for form fields with errors:
ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  "<div class=\"fieldWithErrors\">#{html_tag}</div>" 
end

# You may turn off inline form error messages if they mess up your form layouts
config.action_view.field_error_proc = Proc.new{ |html_tag, instance| html_tag }