How To Limit The Cart Quantity Of Discounted Product

Introduction

1. In this tutorial, you will learn how to control the quantity of the specific set of products based on a logical condition
2. Here in this tutorial, we considered the condition if the discounted product added more than our defined quantity it will notify/alert the end-user but once you understand the concept the same can be applied in different scenarios with little modifications
3. Through this process, you will learn the following from the technical/development side
   1. woocommerce_add_to_cart_validation filter hook
   2. woocommerce_update_cart_validation filter hook
   3. wc_add_notice WooCommerce notice function
   4. wc_get_product function to get product instance
   5. get_regular_price product instance method
   6. get_sale_price product instance method
   7. generate_cart_id cart instance method
   8. find_product_in_cart cart instance method
   9. get_cart_item cart instance method
4. Sample notice

Add to cart validation

1. Line 2: we defined a named constant MAX_QUANTITY where it helps how much discounted product a user can add to a cart
2. Line 4: we called the filter hook function add_filter with the tag woocommerce_add_to_cart_validation for the function validate_discounted_prdct_qty_limitation
3. The filter woocommerce_add_to_cart_validation is responsible for doing validation while adding the product to the cart
4. Line 19: Lets we see the inner working of the hooked function validate_discounted_prdct_qty_limitation
5. The hooked function has three arguments
   1. $passed Boolean value to pass or fail the validation
   2. $product_id an integer value which is the product id that is the primary key of that product
   3. $quantity an integer which has the intended quantity of the product
6. Line 20: we call the function wc_get_product where we passed our product id which returns the right product object by making using of a factory pattern class WC_Product_Factory which return any one of the product object types WC_Product_Simple, WC_Product_Grouped, WC_Product_External, WC_Product_Variable, etc.,
7. Line 22: we get the products parent id if the product doesn’t have a parent which return 0 else it returns the parent product id
8. Line 23: if the product id isn’t empty (possible for variable products child product or for any custom product type) then assign the variable $product_id with the product’s parent id
9. Line 27: we call our validator function validate_product_qty and return it
10. On the shop page, when the threshold limit is crossed instead of showing the notice on shop page WooCommerce redirects to a specific product page and alerts the notice which is the default flow of the ajax add to cart functionality

Update cart validation

1. Line 3: we hooked our custom function on_update_cart_limit_qty to the hook tag woocommerce_update_cart_validation which is responsible for update cart validation
2. Therefore this custom function solely control the WooCommerce cart page validation and previous section we explored the hook woocommerce_add_to_cart_validation which can be triggered on the shop page or product page
3. Line 19: the function on_update_cart_limit_qty is a simple one-line function which calls the validation function validate_product_qty
4. Lets we see the arguments of this function
   1. $passed Boolean value to pass or fail the validation
   2. $cart_item_key string value, to be clear WooCommerce generated MD5 which is used as a key for every cart item in the cart
   3. $values array value, currently updated product cart item properties
   4. $quantity int value, which has the product’s updated quantity

Show validate message

1. Line 12: you may be surprised by seeing this function as earlier we saw that this function as the validation function but here it just the message notification function
2. The reason for this is we abstracted our validation logic in other business logic function is_discounted_prdct_qty_exceeded so you can reuse all the function up to we discussed (including this) without modifying anything and interpolate your desired validation business logic function into the if condition or even if you’re using these functions as object-oriented methods then you can inject the validation business logic as dependency injection (DI)
3. This function accepts three arguments they are
   1. $product_id int value, which holds the product’s id (Primary Key)
   2. $quantity int value, validating product quantity which can be verified against our MAX_QUANTITY constant
   3. $action_type string value, a switcher key which accepts either add or update as value depends on this value we slightly modify the quantity calculation logic in the function is_discounted_prdct_qty_exceeded
4. Line 13: here whatever argument values we received are directly passed to the validation business logic function is_discounted_prdct_qty_exceeded
5. Line 15: the variable $output_string holds the string returned by the WordPress function wp_sprintf
6. Line 21: the WooCommerce function wc_add_notice is used to display the validation message
7. If product quantity exceeded the threshold limit then we return false else true

Validation business logic

1. Lets we see about the helper function is_discounted_prdct_qty_exceeded
2. Line 13: We passed the argument $product_id to the WooCommerce function wc_get_product which return the WC_Product object, if the product exists for the given product id else, return null if not found and if anything wrong return false
3. Line 14: we call the method get_regular_price of the WC_Product object variable $product_obj this method is responsible to return the product’s regular price i.e. actual price
4. Line 15: the method get_sale_price is similar to get_regular_price but it returns the discounted price
5. Line 17: we called the cart instance method generate_cart_id which return the unique cart id of the passed product id
6. Line 18: the cart instance method find_product_in_cart return the cart item key if the product is in the cart else return an empty string, this method is used to find whether our product is already in the cart or not
   1. We will use this variable in if condition while adding a product to the cart we will see the purpose of this variable later
7. Line 20: this conditional logic is used to check whether our product is the discounted one or not
   1. ! empty( $sale_price ) here we check whether our sale price is not empty
   2. (float) $sale_price < (float) $regular_price here we check if the sale price is lesser than the regular price to decide whether the product is discounted or not
8. Line 23: this conditional block is executed only when we are adding the product to the cart and the same product already present in the cart
   1. 'add' === $action_type here we checking is our action type is add if true means the next expression will be executed
   2. ! empty( $find_product_in_cart ) here we check if the product already in the cart
   3. The reason for checking this condition is considering like this if the product which we are adding is already in the cart with a quantity 2 and the currently adding product quantity is 3 means we need to check the sum of both cart and added product quantity which will be 5 against our threshold value 3 instead of that if we check only added quantity with the threshold value it will pass but it’s wrong therefore we summed up both added product and cart quantity
9. Line 26: fetching a particular product’s data in the cart using the cart instance method get_cart_item using the cart item key which we get by passing the variable $cart_id
10. Line 27: the variable $cart_item holds an array which has all the details of a particular product in the cart, we fetching the product’s cart quantity $cart_item['quantity'] and sum with the add to cart quantity finally assign the total sum to the variable $quantity
11. Line 30: conditional check to check whether we crossed the threshold quantity if so then return true
12. Line 36: if the conditions are fails then return false to indicate threshold is either not reached or not applicable for this product

Conclusion

1. In this tutorial, you learned how to control the quantity of the products in the cart
2. You can find the full source code in the GitHub