Fat model

product.rb

class Product < ActiveRecord::Base

  has_many :orders
  validates :name, presence: true
  validates :stock_level, presence: true
  validates :min_stock_level, presence: true
  
  before_validation :strip_name
  before_save :notify_if_low_on_stock
  after_save :notify_product_created
  
  def low_stock?
    stock_level <= min_stock_level
  end

  private

  def strip_name
    @name.strip! if @name.present?
  end
  
  def notify_if_low_on_stock
    if old_stock_level >= min_stock_level && new_stock_level < min_stock_level
      Notification.create message: "Buy more #{name}"
    end
  end

  def old_stock_level
    (stock_level_change.first || 0)
  end

  def new_stock_level
    stock_level_change.last
  end
  
  def notify_product_created
    Notification.create message: "Product #{name} created"
  end
end