ActsAsParentOf

If you are on Rails 2.3 or above, use Rails' nested attributes feature.

If you are on Rails 2.2 or below, keep reading.

This is lifted straight out of Advanced Rails Recipe 13, "Handle Multiple Models In One Form", by Ryan Bates.

For those using the recipe, you can save yourself some typing by adding the acts_as_parent_of declaration to your parent model. For example:

class Project < ActiveRecord::Base
  acts_as_parent_of :tasks, :budget
end

You'll still need to write your views and controller as explained in the recipe.

has_many versus has_one

Although the recipe assumes your parent has many children, acts_as_parent_of also handles the case where the parent has one child (e.g. :budget in the example above). Just remember in your controller's update method to set the child's attributes to nil rather than {}. For example:

class ProjectController < ApplicationController
  def update
    params[:project][:existing_task_attributes] ||= {}
    params[:project][:budget_attributes] ||= nil

    # your normal update code etc...
  end
end

acts_as_parent_of

README.markdown

ActsAsParentOf

has_many versus has_one