Example of nested attributes from belongs_to side of association.

_form.html.erb

<% form_for @event do |f| %>
  <%= f.error_messages %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>

	<% f.fields_for :category do |category_form| %>
  
	  <%= category_form.label :name %>
	  <%= category_form.text_field :name %>
	  <%= category_form.text_field :color %>
  
	  <% unless category_form.object.new_record? %>
	    <%= category_form.check_box '_delete' %>
	    <%= category_form.label '_delete', 'Remove' %>
	  <% end %>
  
	<% end %>

  <p>
    <%= f.label :start_at %><br />
    <%= f.datetime_select :start_at %>
  </p>
  <p>
    <%= f.label :end_at %><br />
    <%= f.datetime_select :end_at %>
  </p>
  <p><%= f.submit "Submit" %></p>
<% end %>

category.rb

class Category < ActiveRecord::Base
  has_many :events
  
  validates_presence_of :name
end

event.rb

class Event < ActiveRecord::Base
  belongs_to :category
  
  accepts_nested_attributes_for :category, :reject_if => :all_blank, :allow_destroy => true
end

events_controller.rb

class EventsController < ApplicationController
  def index
    @events = Event.all
  end
  
  def show
    @event = Event.find(params[:id])
  end
  
  def new
    @event = Event.new
    @event.build_category
  end
  
  def create
    @event = Event.new(params[:event])
    if @event.save
      flash[:notice] = "Successfully created event."
      redirect_to @event
    else
      render :action => 'new'
    end
  end
  
  def edit
    @event = Event.find(params[:id])
  end
  
  def update
    @event = Event.find(params[:id])
    if @event.update_attributes(params[:event])
      flash[:notice] = "Successfully updated event."
      redirect_to @event
    else
      render :action => 'edit'
    end
  end
  
  def destroy
    @event = Event.find(params[:id])
    @event.destroy
    flash[:notice] = "Successfully destroyed event."
    redirect_to events_url
  end
end