ruby script/plugin install http://dvisionfactory.googlecode.com/svn/rails/plugins/related_select_forms/

The usage is trivial. First create a usual select (or collection_select) which will act as "parent". Afterwards you can create one or many dependent child(ren) select(s) by calling:

    related_collection_select(
        object, method, parent_select_tag, collection, 
        value_method, text_method, reference_method, 
        options = {}, html_options = {})

Arguments:
===========

* 'object', 'method', 'collection', 'value_method', 'text_method', 'options' & 'html_options' are used exactly the same way as in the standard collection_select helper method.
* 'parent_select_tag' specifies, as the name says, the parent select tag; argument can be passed as an array [:parent_object, :method] or directly as string referencing the tag id (e.g. "parent_object_method")
* Parameter 'reference_method' specifies the method that is used to get a reference to parent selection.

Additionally the 'options' argument can include a ':selected' attribute, that will override the default pre-selection behaviour (which uses to call '@object.method' to determine the to be selected option).

Example: Two related select forms
==================================

# tables

    car_companies: id, name
    car_models:    id, name, car_company_id
  
# view

    <%= collection_select(
          :car_company, :id, CarCompany.find(:all), :id, :name) %>
    <%= related_collection_select(
          :car_model, :id, [:car_company, :id], 
            CarModel.find(:all), :id, :name, :car_company_id) %>

The code above will create two drop-down select tags. The 1st allows the selection of a car company. Based on this decision the 2nd select tag shows company specific car models.


Example: Three related select forms
====================================
The script allows to generate an almost unlimited number of related selects:

# tables

    categories: id, title, parent_id
    some_objects: id, name, category_id

# controller

    @first_col = Category.find(:all, :conditions => 'parent_id IS NULL')
    @second_col = Category.find(:all, :conditions => 'parent_id > 0')
    @third_col = SomeObject.find(:all)
    @some_object = SomeObject.find(123)

# view

    <%= collection_select(:category, :id, @first_col, :id, :title) %>
    <%= related_collection_select(:sub_category, :id, [:category, :id], 
          @second_col, :id, :title, :parent_id) %>
    <%= related_collection_select(:some_object, :id, [:sub_category, :id], 
          @third_col, :id, :name, :category_id, {}, {:size => 6}) %>

This example creates three related select tags. The 1st shows 'categories', the 2nd 'sub-categories' and the 3rd 'some_objects'. Because an '@some_object' instence variable is defined, item with id='123' will be pre-selected automatically in the 3rd select tag. The 1st & 2nd select forms will also berecursively adapted automatically.

There is currently no advanced documentation for this plugin.