Have a product in WooCommerce take its inventory from another product

index.php

<?php
/**
 * Variations Pull From Product Inventory
 *
 * @wordpress-plugin
 * Plugin Name:       Variations Pull From Product Inventory
 * Description:       A WooCommerce plugin to allow variations to pull from another product's inventory.
 * Version:           1.3.0
 * Requires at least: 5.2
 * Requires PHP:      7.2
 * Author:            Jesse Conner
 * License URI:       http://www.gnu.org/licenses/gpl-3.0.txt
 *
 * @package Variations Pull From Product Inventory
 */


/**
 * Allows for products that pull from the inventory of other products.
 *
 * @since 1.0.0
 */
class Pull_From_Other_Inventory {

    private const SOURCE_META_FIELD = 'my_source_meta_field';
    private const AMOUNT_META_FIELD = 'my_amount_meta_field';

	/**
	 * Instantiate the hooks and filters of our class
	 *
	 * @return void
	 * @since 1.0.0
	 */
	public function __construct() {

		add_action( 'woocommerce_product_after_variable_attributes', array( $this, 'variation_settings' ), 10, 3 );
		add_action( 'woocommerce_save_product_variation', array( $this, 'save_fields' ), 10, 2 );
		add_action( 'woocommerce_product_options_general_product_data', array( $this, 'product_settings' ) );
		add_action( 'woocommerce_process_product_meta', array( $this, 'save_fields' ), 10, 1 );
		add_action( 'woocommerce_thankyou', array( $this, 'update_stock' ) );
		add_action( 'woocommerce_product_set_stock', array( $this, 'sync_stock' ), 10, 1 );
		add_action( 'woocommerce_check_cart_items', array( $this, 'validate_stock' ), 10 );
	}

	/**
	 * Add the source product settings to variations
	 *
	 * @param WP_Query $loop the existing query loop.
	 * @param WC_Variation $variation_data not actually used, but needed for the hook.
	 * @param WC_Variation $variation the variation object to show the settings for.
	 *
	 * @return void
	 * @since 1.2.0
	 */
	public function variation_settings( $loop, $variation_data, $variation ) {

		$source = get_post_meta( $variation->ID, self::SOURCE_META_FIELD, true );
		$amount = get_post_meta( $variation->ID, self::AMOUNT_META_FIELD, true );

		$source_options = array();
		$args           = array(
			'post_type'      => 'product',
			'posts_per_page' => 1000,
			'meta_query'     => array(
				array(
					'key'     => '_stock',
					'value'   => 0,
					'compare' => '>',
					'type'    => 'NUMERIC',
				),
			),
		);

		$loop = new WP_Query( $args );

		if ( ! $loop->have_posts() ) {
			$source_options[0] = 'None Available';
		} else {
			$source_options[0] = 'None';
		}

		while ( $loop->have_posts() ) :
			$loop->the_post();
			global $product;
			$source_options[ get_the_id() ] = get_the_title();
		endwhile;

		wp_reset_postdata();

		?>
        <div class="option_group">
			<?php

			woocommerce_wp_select(
				array(
					'id'            => self::SOURCE_META_FIELD,
					'label'         => 'Takes inventory from:',
					'value'         => $source,
					'options'       => $source_options,
					'wrapper_class' => 'form-field-wide',
				)
			);

			woocommerce_wp_select(
				array(
					'id'            => self::AMOUNT_META_FIELD,
					'label'         => 'Quantity to take:',
					'value'         => $amount,
					'options'       => array(
						'0'  => '0',
						'1'  => '1',
						'2'  => '2',
						'3'  => '3',
						'4'  => '4',
						'5'  => '5',
						'6'  => '6',
						'7'  => '7',
						'8'  => '8',
						'9'  => '9',
						'10' => '10',
					),
					'wrapper_class' => 'form-field-wide',
				)
			);

			wp_nonce_field( 'save', 'pull-from-inventory' );

			?>

        </div>
		<?php
	}

	/**
	 * Add the source product settings to simple products
	 *
	 * @return void
	 * @since 1.2.0
	 */
	public function product_settings() {

		$product = new WC_Product( get_the_ID() );

		if ( $product->is_type( 'variable' ) ) {

			// Only show on simple products to avoid a lot of confusion.
			return;
		}

		$source = get_post_meta( get_the_ID(), self::SOURCE_META_FIELD, true );
		$amount = get_post_meta( get_the_ID(), self::AMOUNT_META_FIELD, true );

		$source_options = array();
		$args           = array(
			'post_type'      => 'product',
			'posts_per_page' => 1000,
			'meta_query'     => array(
				array(
					'key'     => '_stock',
					'value'   => 0,
					'compare' => '>',
					'type'    => 'NUMERIC',
				),
			),
		);

		$loop = new WP_Query( $args );

		if ( ! $loop->have_posts() ) {
			$source_options[0] = 'None Available';
		} else {
			$source_options[0] = 'None';
		}

		while ( $loop->have_posts() ) :
			$loop->the_post();
			$source_options[ get_the_id() ] = get_the_title();
		endwhile;

		wp_reset_postdata();

		?>
        <div class="option_group">
			<?php

			woocommerce_wp_select(
				array(
					'id'            => self::SOURCE_META_FIELD,
					'label'         => 'Takes inventory from:',
					'value'         => $source,
					'options'       => $source_options,
					'wrapper_class' => 'form-field-wide',
				)
			);

			woocommerce_wp_select(
				array(
					'id'            => self::AMOUNT_META_FIELD,
					'label'         => 'Quantity to take:',
					'value'         => $amount,
					'options'       => array(
						'0'  => '0',
						'1'  => '1',
						'2'  => '2',
						'3'  => '3',
						'4'  => '4',
						'5'  => '5',
						'6'  => '6',
						'7'  => '7',
						'8'  => '8',
						'9'  => '9',
						'10' => '10',
					),
					'wrapper_class' => 'form-field-wide',
				)
			);

			wp_nonce_field( 'save', 'pull-from-inventory' );
			?>

        </div>
		<?php
	}

	/**
	 * Save the product source settings
	 *
	 * @param int $id the id of the product whose meta needs updating.
	 *
	 * @return void
	 * @since 1.0.0
	 */
	public function save_fields( $id ) {

		if ( isset( $_POST[self::SOURCE_META_FIELD], $_POST[self::AMOUNT_META_FIELD] ) && check_admin_referer( 'save', 'pull-from-inventory' ) ) {

			$source = sanitize_meta( self::SOURCE_META_FIELD, wp_unslash( $_POST[self::SOURCE_META_FIELD] ), 'post', 'product' );
			$amount = sanitize_meta( self::AMOUNT_META_FIELD, wp_unslash( $_POST[self::AMOUNT_META_FIELD] ), 'post', 'product' );

		} else {
			return;
		}

		update_post_meta( $id, self::SOURCE_META_FIELD, esc_attr( $source ) );
		update_post_meta( $id, self::AMOUNT_META_FIELD, esc_attr( $amount ) );
	}

	/**
	 * Updates the stock of source items on order marked as complete
	 *
	 * @param int $order_id The id of the order that affects the source stock.
	 *
	 * @return void
	 * @since 1.0.0
	 */
	public function update_stock( $order_id ) {

		if ( get_post_meta( $order_id, 'stock_updated', true ) ) {
			return;
		}

		$order    = wc_get_order( $order_id );
		$products = $order->get_items();

		foreach ( $products as $prod ) {

			if ( $prod['variation_id'] ) {
				$id = $prod['variation_id'];
			} else {
				$id = $prod['product_id'];
			}

			if ( ! $id ) {
				continue;
			}

			$source = get_post_meta( $id, self::SOURCE_META_FIELD, true );
			$amount = get_post_meta( $id, self::AMOUNT_META_FIELD, true ) * $prod['quantity'];

			if ( $source > 0 && $amount > 0 ) {
				wc_update_product_stock( $source, $amount, 'decrease' );
			}
		}

		update_post_meta( $order_id, 'stock_updated', true );
	}

	/**
	 * Updates the stock of the child products when a source has been updated
	 *
	 * @param WC_Product $source_product the product object that needs its children updated.
	 *
	 * @return void
	 * @since 1.0.0
	 */
	public function sync_stock( $source_product ) {

		$args = array(
			'post_type'      => array(
				'product',
				'product_variation',
			),
			'posts_per_page' => 100000,
			'meta_query'     => array(
				array(
					'key'     => self::SOURCE_META_FIELD,
					'value'   => $source_product->get_id(),
					'compare' => '=',
					'type'    => 'NUMERIC',
				),
			),
		);

		$loop = new WP_Query( $args );
		while ( $loop->have_posts() ) :
			$loop->the_post();

			$unit_qty = get_post_meta( get_the_id(), self::AMOUNT_META_FIELD, true );
			$qty      = $source_product->get_stock_quantity();
			$amount   = floor( $qty / $unit_qty );

			if ( ! $amount ) {
				$amount = 0;
			}

			wc_update_product_stock( get_the_ID(), $amount );
		endwhile;

		wp_reset_postdata();
	}

	/**
	 * Validates the stock availability at checkout
	 *
	 * @return void
	 * @since 1.1.0
	 */
	public function validate_stock() {

		$totals = array();

		foreach ( WC()->cart->cart_contents as $cart_content_product ) {

			$id = $cart_content_product['variation_id'] > 0 ? $cart_content_product['variation_id'] : $cart_content_product['product_id'];

			$source = get_post_meta( $id, self::SOURCE_META_FIELD, true );
			$amount = get_post_meta( $id, self::AMOUNT_META_FIELD, true );

			if ( $source && $amount ) :

				// Inventory belongs to a source.
				$qty               = $cart_content_product['quantity'];
				$totals[ $source ] += $amount * $qty;


			endif;
		}

		foreach ( $totals as $controlled_item => $amount ) {

			$source_product = wc_get_product( $controlled_item );

			if ( $amount > $source_product->get_stock_quantity() ) {

				wc_add_notice( '<strong>There is not enough stock to complete the purchase</strong>', 'error' );

			}
		}
	}
}

// Make sure WooCommerce is active before instantiating the plugin.
if ( in_array( 'woocommerce/woocommerce.php', apply_filters( 'active_plugins', get_option( 'active_plugins' ) ), true ) ) {
	// Finally, instantiate our plugin class.
	new Pull_From_Other_Inventory();
}