[x] Add a single product to an order (using order_products). Prevent duplication on ("orderId", "productId") pair. If product already exists on order, increment quantity and update price.
[x] PATCH /order_products/:orderProductId (**)
[x] Update the quantity or price on the order product
[x] DELETE /order_products/:orderProductId (**)
[x] Remove a product from a order, use hard delete
[ ] Add a single product to an order (using order_products). Prevent duplication on ("orderId", "productId") pair. If product already exists on order, increment quantity and update price.
[ ] PATCH /order_products/:orderProductId (**)
[ ] Update the quantity or price on the order product
[ ] DELETE /order_products/:orderProductId (**)
[ ] Remove a product from a order, use hard delete
[x] POST /orders/:orderId/products (**)
[x] PATCH /order_products/:orderProductId (**)
[x] DELETE /order_products/:orderProductId (**)