rails-join-usage-example

# payment_orders join orders
PaymentOrder.joins(:orders)

# payment_orders join orders, payment_orders join gift_packs
PaymentOrder.joins(:orders).joins(:gift_pack)

# payment_orders join orders, orders join products
PaymentOrder.joins(orders: :products)

# payment_orders join orders, orders join products, products join product_items
PaymentOrder.joins(orders: {products: :product_items})