ref
https://linear.app/tryghost/issue/ENG-1251/support-escalation-re-offers-not-tracking
- Offer Redemptions were not being persisted in the database for
existing free members who upgrade to a paid plan with an offer, which
resulted in inaccurate offer redemption counts. This made it difficult
to assess the performance of an offer.
- Previously, Ghost recorded an offer redemption in the DB in response
to the `SubscriptionCreatedEvent`, under the assumption that the offer
details would be included in this event. This assumption was valid for
brand new members starting a subscription with an offer, but not for
existing free members upgrading to a paid plan with an offer.
- For existing free members, the subscription is first stored in Ghost
in response to the `customer.subscription.created` Stripe webhook. At
this point, the offer/discount is not attached to the subscription, so
the `SubscriptionCreatedEvent` triggers without the offer information,
and the offer redemption is not recorded. After the
`checkout.session.completed` webhook is received (which _does_ include
the offer details), the subscription is updated in Ghost, but the Offer
Redemption is not stored.
- For brand new members, the `customer.subscription.created` webhook
no-ops, because the member and Stripe Customer don't exist yet.
Therefore, the subscription is first created in Ghost in response to the
`checkout.session.completed` webhook, which _does_ include the offer
information, so the offer information is included in the
`SubscriptionCreatedEvent` and the offer redemption is recorded as
expected.
- This change adds a new `OfferRedemptionEvent`, which triggers
either: (1) when a new subscription is created with an offer (as in the
case of a brand new member), or (2) when an existing subscription is
first updated to include an offer (as in the case of an existing free
member upgrading with an offer). The Offer Redemption is then persisted
in the DB in response to the `OfferRedemptionEvent` rather than the
`SubscriptionCreatedEvent`.
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