DistroPropaganda/code/rails32/depot_s/app/models/product.rb

#---
# Excerpted from "Agile Web Development with Rails",
# published by The Pragmatic Bookshelf.
# Copyrights apply to this code. It may not be used to create training material,
# courses, books, articles, and the like. Contact us if you are in doubt.
# We make no guarantees that this code is fit for any purpose.
# Visit http://www.pragmaticprogrammer.com/titles/rails4 for more book information.
#---
class Product < ActiveRecord::Base
  has_many :line_items
  has_many :orders, through: :line_items
  #...

  before_destroy :ensure_not_referenced_by_any_line_item

  attr_accessible :description, :image_url, :price, :title
  validates :title, :description, :image_url, presence: true
  validates :price, numericality: {greater_than_or_equal_to: 0.01}
# 
  validates :title, uniqueness: true
  validates :image_url, allow_blank: true, format: {
    with:    %r{\.(gif|jpg|png)\Z}i,
    message: 'must be a URL for GIF, JPG or PNG image.'
  }
  validates :title, length: {minimum: 10}

  private

    # ensure that there are no line items referencing this product
    def ensure_not_referenced_by_any_line_item
      if line_items.empty?
        return true
      else
        errors.add(:base, 'Line Items present')
        return false
      end
    end
end