Spree Promotions

A promotion is "if X is true about the cart, do Y." Spree breaks that into:

Promotion           — the campaign (name, dates, code, usage limits)
  ├── PromotionRule × n        — eligibility checks: "if X is true"
  ├── PromotionAction × n      — what to do: "create a discount", "free shipping"
  │     └── Calculator         — how much (flat $, %, per-item, etc.)
  └── PromotionCategory        — admin grouping

A promo can have multiple rules (ANDed or ORed via match_policy) and multiple actions (all fire when eligible).

Promotion attributes