Spree Promotions

A promotion (promo_) is a campaign: rules decide whether it applies, actions decide what it does. What lands on a cart/order is a discount (Spree::Discount, disc_) — a money row that survives the promotion being edited or deleted.

Promotion (belongs_to :store; kind coupon_code|automatic; starts_at/expires_at; usage_limit; match_policy all|any)
  ├── PromotionRule (prorule_, STI)      — eligible?(cart_or_order)
  ├── PromotionAction (pact_, STI)       — discount_scope + compute_amount, or a side effect in perform
  │     └── Calculator (calc_)            — how much (FlatRate, FlatPercentItemTotal, PercentOnLineItem, …)
  └── CouponCode (coupon_) × n           — multi-code promotions; state unused|used
Discount — kind 'promotion' | 'manual'; attached to exactly one LineItem or Fulfillment; amount ≤ 0

How promotions apply

Every cart change runs Spree::Carts::RecalculateTotals, which runs Spree::Adjusters::Promotion: