Add foreign key constraint between order_promotions and promotions

[mamhoff]

Summary

When we delete a promotion, the associated order_promotions should also disappear.

Checklist

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.
• 📑 I have documented new code with YARD.
• ✅ I have added automated tests to cover my changes.

[jarednorman]

Couldn't this cause issues with historical orders?

[mamhoff]

Couldn't this cause issues with historical orders?

Solidus' promotion system won't recalculate promotion adjustments after orders are completed. So adjustments for historical orders would not change. One would not be able to trace why a certain promotion applied to an order, but once you delete the promotion, chances are you don't really care?

Currently the state of deletion in the promotion world is generally weird: PromotionAction records are soft-deleted, but promotions and promotion rules are not.

This is just to stop garbage data from being around - because what good is it to know I have an order_promotion record where the promotion in question has been hard deleted. If the order_promotion record contained a promotion code id, that's also gone.

[jarednorman]

Right, that makes sense.

[codecov]

Codecov Report

Merging #5469 (f2649f7) into main (00031a2) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5469   +/-   ##
=======================================
  Coverage   88.93%   88.93%           
=======================================
  Files         622      622           
  Lines       14909    14909           
=======================================
  Hits        13259    13259           
  Misses       1650     1650           

Files	Coverage Δ
core/app/models/spree/promotion.rb	97.58% <100.00%> (ø)

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