I like most things provided from the Guru site... Just make sure to read the disclaimer:
While this solution gets you as close as possible to populating the correct order guide used to generate a request, you may still notice some false positives due to system behavior we can't control. Any time people click on an order guide item in the catalog (even if they don't end up completing the order guide), the order guide is populated on the cart and will be populated on the next request. Another issue is that people can work through an order guide and then delete certain contents of the request on the confirmation screen, and then go back to the catalog and add more items unrelated to the order guide. Unfortunately, there's not any way to guarantee that the order guide will be populated correctly 100% of the time because of these factors.
The script below mitigates some of these issues by checking to make sure that at least one of the items from the order guide rule base is included in the order if we are populating the Order guide field on the request. It also does a second check to verify that any order guide rule base entry with NO condition (meaning it always ends up being added to the order guide) MUST be included in the order.
Even with these additional checks in place, the system behavior might still result in requests where somebody clicked on an order guide, then ordered one of the items in the guide as a standalone item instead. The best way to guard against this is to make sure you have at least one item with no conditions in the rule base, ensuring that it will always have to be part of the order if we're going to populate an order guide.
