Improve the demo experience

rails-app/app/models/application_record.rb

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+class ApplicationRecord < ActiveRecord::Base
+  self.abstract_class = true
+end

rails-app/app/models/customer.rb

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+class Customer < ApplicationRecord
+  # Include default devise modules. Others available are:
+  # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
+  devise :database_authenticatable, :registerable,
+         :recoverable, :rememberable, :validatable
+
+  has_many :products, through: :purchases
+end

rails-app/app/models/product.rb

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+class Product < ApplicationRecord
+  has_many :customers, through: :purchases
+end

rails-app/app/models/purchase.rb

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+class Purchase < ApplicationRecord
+  validates :sale_type, :inclusion => { :in => %w{rented bought} }
+  validates :quantity, numericality: { greater_than: 0 }
+end

rails-app/app/models/user.rb

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+class User < ApplicationRecord
+  # Include default devise modules. Others available are:
+  # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable
+  devise :database_authenticatable, :registerable,
+         :recoverable, :rememberable, :validatable
+  
+  has_many :products
+end