Intervention gives you a number of handy FormBuilder methods to make handling relationships within your forms easier.

Ugly, out of fashion, and unhip:

<%= form.select :country, [Country.new(:name => "Outer Space")] + Country.find(:all) %>
<%= form.select :plan, Plan.order(:name).free.all %>
<%= form.select :priority, controller.current_user.account.priorities.all %>

New, stylish, and clean:

<%= form.select_field :country, :from => :eu_countries, :include_blank => "Outer Space" %>
<%= form.select_field :plan, :type => :free %>
<%= form.select_field :priority %>

The :from attribute defines a method name which returns the array selections to use. These can be name/value pairs as a Hash, as a nested Array, or as any object which responds to #name and #id. The :from attribute defaults to the underscored version of the class name of the relationship, if there is one.

Dropdowns are defined in the file lib/dropdowns.rb. Any options passed to select_field are also passed to the corresponding method in this class.

class Dropdowns < Intervention::Dropdowns

  def eu_countries(options = {})
    Rails.cache("eu_countries_by_name") { Country.order(:name).eu.all }
  end

  def plans(options = {})
    if options.delete(:type) == :free
      Plan.order(:name).free.all
    else
      Plan.order(:name).all
    end
  end

  def priorities
    controller.current_user.account.priorities.all
  end

end

Why? Its damn easy to test, great for re-use, easily cacheable, and your views won't be littered with as much confusing ERB code.

Almost every application we make has some kind of expandable list. "Add Another", "Remove This", its all very repetitive.

<% form.field_array_for :tasks, :blank => 1 do |task| %>
  <%= task.text_field :name %>
  <%= task.select_field :priority %>
  <%= task.delete_link "Remove This" %>
<% end %>

This helper renders all existing tasks, and provides the ability to add another one, and even gives you a blank one for free.