rails5-depot/app/models/product.rb

class Product < ApplicationRecord
  has_many :line_items

  before_destroy :ensure_not_referenced_by_any_line_item

  validates :title, :description, :image_url, :price, presence: true
  validates :price, numericality: {greater_than_or_equal_to: 0.01}
  validates :title, uniqueness: true, length: { minimum: 5, maximum: 30 }
  validates :image_url, allow_blank: true, format: {
    with: %r{\.(gif|jpg|png)\Z}i,
    message: 'must be URL for GIF, JPG or PNG image.'
  }

  private
    def ensure_not_referenced_by_any_line_item
      unless line_items.empty?
        errors.add(:base, 'Line Items present')
        throw :abort
      end
    end
end