[FR] Cache part requirements #11211

Problem statement

Part "requirement" data is expensive to compute dynamically - and is only becoming more so.

Functionally this data does not change frequently but it used in many "on the fly" database annotations.

Requirement data includes:

• Number required for sales order
• Number required for build orders
• Number allocated to sales orders
• Number allocated to build orders
• Number incoming from purchase orders

These quantities should include:

• Variant quantities (for template parts)
• Substitute options (for BOM lines)

Suggested solution

• Add a new table with a 1:1 relationship to the "part" model which stores this requirements information for rapid access.
• Significantly simplify API calls which currently compute this on the fly
• De-couple business logic from database annotations in the queryset
• Simplify implementation of new features which adjust the requirements for a part

Such a feature would also require that we add a number of hooks to re-calculate the requirements data when the underlying instances change. However we can at least point to a single function which provides a single "source of truth" for each part.