Performance improvements: rendering the Subscription Relationship column and checking if order is a renewal, switch etc.

[mattallan]

Part of #731

Description

When a merchant visits the WooCommerce > Orders list table, WooCommerce Subscriptions adds a custom "Subscription Relationship" column that displays an icon based on what type of order it is:

1. Parent (initial subscription) Order
2. Renewal Order
3. Resubscribe Order

As reported in #731, our render_contains_subscription_column_content() function currently performs unnecessary queries which impacts overall site performance and performance when viewing/managing orders via the WooCommerce > Orders table. After looking at this function, I noticed a couple of issues that we should address:

1. wcs_order_contains_subscription() unnecessarily loads subscription objects into memory.
2. We already have the Order object when rendering the row/column so we should use it instead of passing the order ID around.
3. There's no efficient helper function to check if an order is a parent order.
4. All of our wcs_order_contains_{order_type}() functions are doing a lot when all we really need to do is check for meta on an order.

This PR targets all of these issues:

• dd1299a - Introduces a new wcs_is_parent_order( $order ) function which runs a much more performant query to check if an order is a parent order.
• 44321ec - Refactors all of our wcs_order_contains_renewal(), wcs_order_contains_resubscribe() and wcs_order_contains_switch() functions to use our WCS_Related_Order_Store class to check if the order belongs to a renewal, resubscribe and switch, without having to load subscription objects into memory.
• 976c961 - Refactors the render_contains_subscription_column_content() to call the updated/more performant wcs_order_contains_{order_type}( $order ) functions directly instead of going through wcs_order_contains_subscription( $order_id, {order_type} )
• fc91135 - Updates wcs_order_contains_subscription() to use more performant method for checking parent order.

How to test this PR

Important

The changes in this PR impact potentially a lot of critical subscription flows. We should make sure these functions are heavily unit tested and test our critical flows to assess impact.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• Added deprecated functions, hooks or classes to the spreadsheet

[mattallan]

Breakdown of these changes

Checking if an order is renewal/resubscribe

Old

1. Get order object: wc_get_order( $order_id )
2. Get subscription id from meta: $order->get_meta( '_subscription_renewal', false );
3. Load in subscription object: wcs_get_subscription( $subscription_id );
4. Check if $subscription is not empty.

This branch

1. Get subscription id from meta: $order->get_meta( '_subscription_renewal', false );
2. Check if $subscription_id is exists.

Important

As seen, these changes will no longer load a subscription object into memory which means we're no longer checking if the subscription ID stored in _subscription_renewal is actually a valid subscription object. This means with the changes in this PR, wcs_order_contains_renewal() and other similar "contains_x()" functions, will now return true even if the subscription no longer exists.

To determine if this is a bug or not, we should ask ourselves whether an order should revert to a standalone/regular order if the subscription has been deleted 🤔

I can't think of good reason why we should revert an order to a regular order if the linked subscription has been removed and I believe it would still be good to know that this order was reference to a subscription renewal or resubscribe etc.

If we consider this to be a bug, one way we could address this is to hook onto woocommerce_subscription_deleted and delete any relation meta for this subscription. This would prevent us having to load a subscription into memory just to check if the relation is still valid.

Checking if an order is a parent

Old

1. Get order object: wc_get_order( $order_id )
2. Query for subscriptions: wcs_get_orders( [ 'limit' => -1, 'parent' => 1234, 'type' => 'shop_subscription' ] )
3. Load in subscription object: wcs_get_subscription( $subscription_id );
4. Count subscriptions found.

This branch

1. Query for subscriptions: wcs_get_orders( [ 'limit' => 1, 'parent' => 1234, 'type' => 'shop_subscription' ] )