Bare bones e-commerce Rails models

migrations.rb

class CreateTables < ActiveRecord::Migration
  def change
    create_table :products do |t|
      # ...
    end
    create_table :orders do |t|
      # ...
    end
    create_table :order_items do |t|
      t.references :order, index: true, foreign_key: true
      t.references :product, index: true, foreign_key: true
      t.integer :quantity
      # ...
    end
  end
end

order.rb

class Order < ActiveRecord::Base
  has_many :order_items
  has_many :products, through: :order_items
end

order_item.rb

class OrderItem < ActiveRecord::Base
  belongs_to :order
  belongs_to :product
end

product.rb

class Product < ActiveRecord::Base
  has_many :order_items
  has_many :orders, through: :order_items
end