note.rb

class Note < ActiveRecord::Base
  belongs_to :customer
  attr_accessible :content
end
# == Schema Information
#
# Table name: notes
#
#  id          :integer         not null, primary key
#  content     :text
#  customer_id :integer
#  created_at  :datetime        not null
#  updated_at  :datetime        not null
#

notes_controller.rb

class NotesController < ApplicationController

def show
  @note = Note.find(params[:id])
  #@customer = Customer.find(@note.customer_id) # This works commented out? Bwa?

  respond_to do |format|
    format.html
    format.json {render json: @note }
  end
end
  
  # GET /notes/new
  # GET /notes/new.json
  def new
    @note = Note.new
    @customer = Customer.find(params[:customer_id])

    respond_to do |format|
      format.html # new.html.erb
      format.json { render json: @note }
    end
  end

  # GET /notes/1/edit
  def edit
    @note = Note.find(params[:id])
    @customer = Customer.find(@note.customer_id)
  end

  # POST /notes
  # POST /notes.json
  def create
    @customer = Customer.find(params[:customer_id])
    @note = @customer.notes.build(params[:note])

    respond_to do |format|
      if @note.save
        format.html { redirect_to @customer, notice: 'Note was successfully created.' }
        format.json { render json: @note, status: :created, location: @note }
      else
        format.html { render action: "new" }
        format.json { render json: @note.errors, status: :unprocessable_entity }
      end
    end
  end

  # PUT /notes/1
  # PUT /notes/1.json
  def update
    @note = Note.find(params[:id])
    @customer = Customer.find(@note.customer_id)

    respond_to do |format|
      if @note.update_attributes(params[:note])
        format.html { redirect_to @customer, notice: 'Note was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: "edit" }
        format.json { render json: @note.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /notes/1
  # DELETE /notes/1.json
  def destroy
    @note = Note.find(params[:id])
    @note.destroy

    respond_to do |format|
      format.html { redirect_to :back }
      format.json { head :no_content }
    end
  end
end

show.html.erb

<p id="notice"><%= notice %></p>

<p>
  <b>Last updated:</b>
  <%= @note.updated_at %>
</p>

<p>
  <%= @note.content %>
</p>


<%= link_to 'Edit', edit_note_path(@note) %> |
<%= link_to 'Back', customer_path(@customer) %>