Fix #410
CRM_Stripe_Customer::find()
is only called from CRM/Core/Payment/Stripe.php
The returned value is saved as $stripeCustomerId
and then passed to the Stripe API to retrieve the customer record.
Clearly the calling code is expecting to receive a Stripe customer_id
such as cus_123456
rather than the id
column from the civicrm_stripe_customer
table.
This PR corrects CRM_Stripe_Customer::find()
to return the customer_id
and brings it in line with changes already in 6.8