Create 1. Demonstrate ability to use quote, quote item, address, and shopping cart rules in checkout.md

Author: unicoder88

8. Customizing the Checkout Process/1. Demonstrate ability to use quote, quote item, address, and shopping cart rules in checkout.md

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+#  Demonstrate ability to use quote, quote item, address, and shopping cart rules in checkout
+
+Interesting *quote* fields or methods:
+- `ext_shipping_info` - TEXT 64k, existed in M1, not used. Good place to keep shipping API response?
+- `trigger_recollect`
+  * quoteResource.markQuotesRecollectOnCatalogRules - Mark quotes that depend on catalog price rules
+    to be recollected on demand. Called on event `catalogrule_before_apply`. Good way to update carts
+    after "hacking" prices?
+  * quoteResource.markQuotesRecollect - by product IDs. Mark recollect contain product(s) quotes
+    + global plugin after product save, _if price or tier price changed_, trigger recollect
+    + adminhtml event after product save, _if status disabled_.
+- `is_changed` - not used
+  * before quote save - see temporary data-attribute quote.`changed_flag` - not used
+- `orig_order_id` -  not used
+- `items_count` - INT "Display number of items in cart"
+- `items_qty` - FLOAT "Display item quantities", e.g. 1.5 kg, 3 same chairs etc.
+- `converted_at` - not used
+- getItemsByProduct - matching product id and custom options. good to use right after add to cart
+  + quote.addProduct
+  + after withlist aded to cart, finds and changes store id
+  + withlist add all to cart, on error find and remove from cart
+
+Quote extends Model\AbstractExtensibleModel - has support of `extension_attributes`.
+Register extensions in `extension_attributes.xml`:
+```xml
+<extension_attributes for="Magento\Quote\Api\Data\CartInterface">
+    <attribute code="shipping_assignments" type="Magento\Quote\Api\Data\ShippingAssignmentInterface[]" />
+</extension_attributes>
+```
+Use quote repository plugins afterLoad, beforeSave or whenever needed to populate your values.
+Quote does not utilize `custom_attributes` as it is not EAV by nature.
+
+Quote address custom attributes:
+* community \Magento\Quote\Model\Quote\Address\CustomAttributeList - empty, use plugin to add
+* EE \Magento\CustomerCustomAttributes\Model\Quote\Address\CustomAttributeList::getAttributes
+  + customer address attributes + customer attributes
+
+Quote Item shares 2 models, quote item and quote address item.
+
+*Quote item* interesting methods:
+- checkData - called after adding to cart and updating options
+  * quote item.setQty - triggers stock validation
+  * product type instance.checkProductBuyState
+- `custom_price`
+- getCalculationPrice - `custom_price` or converted price
+- isChildrenCalculated - product.getPriceType = CALCULATE_CHILD
+- isShipSeparately - product.getShipmentType = SHIPMENT_SEPARATELY
+- `additional_data` - TEXT
+- getOptions
+- getQtyOptions:
+  * for composite products in cart when sub-products are added, returns array something like
+    `[$subProductId = qty (from option `product_qty_{$subProductId}`), otherSubId = qty, ...]`
+    + used in \Magento\CatalogInventory\Model\Quote\Item\QuantityValidator::validate
+- compare - \Magento\Quote\Model\Quote\Item\Compare::compare - merge items and add qty instead of new item
+  + \Magento\Quote\Model\Quote::updateItem
+- representProduct - compares quote item with some new product, checks product id and custom options
+  + quote.getItemsByProduct
+- compareOptions 
+  + representProduct
+
+## Describe how to modify these models and effectively use them in customizations.
+- All support extension attributes
+- Quote address supports custom_attributes
+
+## Inventory validation
+Qty can be updated from:
+- cart model.updateItems by:
+  * checkout/cart/updatePost with update_cart_action='update_qty'
+- quote.updateItem by:
+  * admin/sales/order/create
+  * advanced checkout
+  * checkout/cart/updateItemOptions
+  * admin/cart_product_composite_cart/update
+  * quote repository.save via Savehandler and CartItemPersister
+
+on qty update:
+- quote item.setQty
+- event `sales_quote_item_qty_set_after`
+- \Magento\CatalogInventory\Observer\QuantityValidatorObserver::execute
+- \Magento\CatalogInventory\Model\Quote\Item\QuantityValidator::validate
+  * all validation errors can set quote.addErrorInfo, quote item.addErrorInfo
+  * check out of stock, parent out of stock
+  * when there's sub products with qty options:
+    + check qty increments
+    + for each sub product - initializer option.initialize
+      \Magento\CatalogInventory\Model\Quote\Item\QuantityValidator\Initializer\Option::initialize
+      - multiply parent qty`*`sub qty, check resulting min sale qty, max sale qty, qty increments, in stock, backorders
+        \Magento\CatalogInventory\Model\StockState::checkQuoteItemQty,
+        \Magento\CatalogInventory\Model\StockStateProvider::checkQuoteItemQty
+  * when no sub products qty:
+    + initializer stock item.initialize
+    \Magento\CatalogInventory\Model\Quote\Item\QuantityValidator\Initializer\StockItem::initialize
+    \Magento\CatalogInventory\Model\StockState::checkQuoteItemQty
+    \Magento\CatalogInventory\Model\StockStateProvider::checkQuoteItemQty
+
+## Add to cart
+cart model - deprecated.
+
+- checkout/cart/add [product, qty, related_product]
+  * cart model.addProduct
+    + filter request here, can register own via argument for \Magento\Checkout\Model\Cart\RequestInfoFilter
+    + by default filtered out `form_key` and `custom_price`
+  * quote.addProduct
+  * product type instance.prepareForCartAdvanced
+  * get same quote item or create new
+  * \Magento\Quote\Model\Quote\Item\Processor::prepare
+    + quote item.addQty
+    + support request.customPrice -> quote item.customPrice
+  * event `checkout_cart_product_add_after`
+  * checkout session.setLastAddedProductId
+  * cart.save
+    + quote collect totals
+    + event `checkout_cart_save_after`
+    + reinitialize state - remove addresses and payments
+  * event `checkout_cart_add_product_complete`
+
+- checkout/cart/updatePost
+  * cart.suggestItemsQty => \Magento\CatalogInventory\Model\StockStateProvider::suggestQty - qty increments, min/max qty
+  * cart.updateItems
+    + events `checkout_cart_update_items_before`, `checkout_cart_update_items_after`
+    + quote item.setQty -- triggers stock validation
+  * cart.save
+
+## Describe how to customize the process of adding a product to the cart.
+- plugin over product type `prepareForCartAdvanced`
+- event `catalog_product_type_prepare_full_options` - custom options in `_prepareOptions`
+- plugin over \Magento\Quote\Model\Quote\Item\Processor::prepare - qty, custom price
+- event `checkout_cart_product_add_after`
+
+## Which different scenarios should you take into account?
+- add to cart from catalog
+- add to cart from wishlist
+- move all wishlist to cart
+- merge quote when existing customer has quote, then shops as a guest and logs in
+- admin create order
+- admin reorder
+- configure added product - change custom options