Get and set custom cart item/product information prior to WooCommerce checkout, and carry those valus over to Order Item Metadata.

woocommerce-custom-cart-item-data.php

<?php

// UPDATE: Stefan from Stack Overflow has explained a better way to handle cart item data.
// See http://stackoverflow.com/a/32327810/470480

// ----------------------

/*
Instructions:

	1) Save data when adding to cart

	When adding an item to your cart, use ld_woo_set_item_data. The best action for this to occur is
	"woocommerce_add_to_cart". The first parameter for this action contains the "cart_item_key",
	which is required for the function.

	  // Save a custom field called "class-date" to the item added to the cart
	  $date = $_REQUEST['class-date'];
	  ld_woo_set_item_data( cart_item_key, "class-date", $date );

	2) Retrieve item data before checkout

	At any point you may retrieve this data using "ld_woo_get_item_data". If you wantto get a list of
	all cart items, use: "WC()->cart->get_cart()". This will return an array. The keys are the
	"cart_item_key" which will be needed.

	3) Display data assigned to the item after the order is complete

	When a user checks out, their "cart" is emptied and what they had purchased becomes an "order"
	instead. At this point, all of the custom item data is stored as item meta using the key
	"_ld_woo_product_data".

	If your custom data should be displayed to customers or administrators (as a regular Item Meta),
	you want to add your own custom meta instead. To do this, use the action
	"woocommerce_add_order_item_meta" and retrieve our custom item data, and add it as item meta
	on your own.

	  // Save custom field as public item meta, viewable on invoices, receipts, and within the dashboard
	  function custom_add_date_order_meta( $item_id, $values, $cart_item_key ) {
	    // Ensure the product supports our custom field
	    if ( !custom_product_has_date( $values['product_id'] ) ) return;

	    // Retrieve the date which was assigned previously
	    $date = woo_get_item_data( $cart_item_key, 'class-date' );

	    // If the date is available, save it as item metadata. Otherwise use a default value (optional).
	    // Note that item meta key can have uppercase characters and spaces. This is not a slug or HTML id.
	    if ( $date ) wc_add_order_item_meta( $item_id, "Class Date", $date );
	    else wc_add_order_item_meta( $item_id, "Class Date", '[error]' );
	  }
	  add_action('woocommerce_add_order_item_meta', 'custom_add_date_order_meta', 10, 3 );

Functions:

	ld_woo_get_item_data( $cart_item_key, [$key, $default] )
		Returns cart item data for the specified cart item. If $key is provided, a single value is returned.
		Otherwise, an array of all cart item data is returned.

	ld_woo_set_item_data( $cart_item_key, $key, $value )
		Sets the data for a cart item by key. Similar to post meta, but based on session.

	ld_woo_remove_item_data( $cart_item_key, [$key] )
		Removes cart item data, a specific key if $key is provided, otherwise the entire cart item's data variable is removed.
		* Called automatically when product is removed from the cart

	ld_woo_convert_item_session_to_order_meta()
		Occurs during checkout, item data is automatically converted to order metadata, stored under the "_ld_woo_product_data"

*/

function ld_woo_get_item_data( $cart_item_key, $key = null, $default = null ) {
	$data = (array)WC()->session->get( '_ld_woo_product_data' );
	if ( empty( $data[$cart_item_key] ) ) {
		$data[$cart_item_key] = array();
	}

	// If no key specified, return an array of all results.
	if ( $key == null ) {
		return $data[$cart_item_key] ? $data[$cart_item_key] : $default;
	}else{
		return empty( $data[$cart_item_key][$key] ) ? $default : $data[$cart_item_key][$key];
	}
}

function ld_woo_set_item_data( $cart_item_key, $key, $value ) {
	$data = (array)WC()->session->get( '_ld_woo_product_data' );
	if ( empty( $data[$cart_item_key] ) ) {
		$data[$cart_item_key] = array();
	}

	$data[$cart_item_key][$key] = $value;

	WC()->session->set( '_ld_woo_product_data', $data );
}

function ld_woo_remove_item_data( $cart_item_key = null, $key = null ) {
	$data = (array)WC()->session->get( '_ld_woo_product_data' );

	// If no item is specified, delete *all* item data. This happens when we clear the cart (eg, completed checkout)
	if ( $cart_item_key == null ) {
		WC()->session->set( '_ld_woo_product_data', array() );

		return;
	}

	// If item is specified, but no data exists, just return
	if ( !isset( $data[$cart_item_key] ) ) {
		return;
	}

	if ( $key == null ) {
		// No key specified, delete this item data entirely
		unset( $data[$cart_item_key] );
	}else{
		if ( isset( $data[$cart_item_key][$key] ) ) {
			unset( $data[$cart_item_key][$key] );
		}
	}
	WC()->session->set( '_ld_woo_product_data', $data );
}
add_filter( 'woocommerce_before_cart_item_quantity_zero', 'ld_woo_remove_item_data', 10, 1 );
add_filter( 'woocommerce_cart_emptied', 'ld_woo_remove_item_data', 10, 1 );

function ld_woo_convert_item_session_to_order_meta( $item_id, $values, $cart_item_key ) {
	// Occurs during checkout, item data is automatically converted to order item metadata, stored under the "_ld_woo_product_data"
	$cart_item_data = ld_woo_get_item_data( $cart_item_key );

	// Add the array of all meta data to "_ld_woo_product_data". These are hidden, and cannot be seen or changed in the admin.
	if ( !empty( $cart_item_data ) ) {
		wc_add_order_item_meta( $item_id, '_ld_woo_product_data', $cart_item_data );
	}
}

add_action( 'woocommerce_add_order_item_meta', 'ld_woo_convert_item_session_to_order_meta', 10, 3 );