cheat sheets.

---
This select box serves two purposes in a view, say a 'new' or 'edit' view:
1. The user can choose a pre-existing object by selecting from a dropdown
   list of the existing objects, rather than typing into an entry field.
2. The user can reference the object by a friendly name rather than by id.

In the 'foobar' Person view, to select a reference to a Person:

  <% form_for([foobar]) do |f| %>
    <%= f.label "Name:" %>
    <%= f.select :person_id, Person.select_list %>

And in the Person model:

  def self.select_list
    self.find(:all).map { |p| [p.name, p.id] }
  end


To choose from a list of a subset of all People:

In the view:
  <%= f.label "Member:" %>
  <%= f.select :person_id, Person.select_member_list %>

And in the Person model:
  def self.select_member_list
    self.find(:all).map { |p| [p.name, p.id] if p.member? }.compact
  end

--
When there is a one-to-many association involved, and you are presenting
a choice from among the existing association objects, there is an easy
collection_select method that the belongs_to association gives you.  As
Ryan Bates discusses in Railscasts episode 057, in the view to create a
Product, to choose its Category, you can use:

  <%= f.collection_select :category_id, Category.all, :id, :name,
    :prompt => "Select a category:" %>

To provide the option to define & choose a new Category when creating a
Product, you can add the Category creation to the Person model and view.
The belongs_to association provides the 'create_[associated model]'
method.

In the view:
  ... Or create a new category:
  <%= f.text_field :new_category_name %>

And in the Person model, define the new attribute (new_category_name),
and a before_save callback to create the new category if entered:

  attr_accessor :new_category_name
  before_save :create_category_from_name
  def create_category_from_name
    create_category(:name => new_category_name) unless \
      new_category_name.blank?