Solution:
To force a specific product to be sold alone, in a separate order, use:
function filter_woocommerce_add_to_cart_validation( $passed, $product_id, $quantity, $variation_id = null, $variations = null ) {
// Product id to bought alone
$product_id_alone = 666;
// Set variable
$flag = false;
// If cart is NOT empty when a product is added
if ( ! WC()->cart->is_empty() ) {
// Generate a unique ID for the cart item
$product_cart_id = WC()->cart->generate_cart_id( $product_id_alone );
// Check if product is in the cart
$in_cart = WC()->cart->find_product_in_cart( $product_cart_id );
// If product is already in cart & product ID added is not equal to product ID alone
if ( $in_cart && ( $product_id != $product_id_alone ) ) {
$flag = true;
// Product ID alone is NOT in cart & product ID added is equal to product ID alone
} elseif( ! $in_cart && ( $product_id == $product_id_alone ) ) {
$flag = true;
}
}
// True
if ( $flag ) {
// Set error message
wc_add_notice( sprintf(
__( 'Product %s must be bought separately', 'woocommerce' ),
$product_id_alone,
), 'error' );
// Passed = false
$passed = false;
}
return $passed;
}
add_filter( 'woocommerce_add_to_cart_validation', 'filter_woocommerce_add_to_cart_validation', 10, 5 );
When adding the product with ID 666:
- If the cart is empty, the product can be added with any quantity.
- If the cart is not empty & the product with ID 666 is NOT in cart, stop.
- If the cart is not empty & the product with ID 666 is in cart, continue.
When a product with a different ID is added:
- If the cart is empty, the product can be added with any quantity.
- If the cart is not empty & the product with ID 666 is NOT in cart, continue.
- If the cart is not empty & the product with ID 666 is in cart, stop.