exports.addPaymentSource = functions.firestore.document('Users/{userId}/paymentSources/{paymentID}').onWrite((change, context) => {
  let newPaymentSource = change.after.data();
  let token = newPaymentSource.token;

  return functions.firestore.document(`Users/${context.params.userId}`).get('customer_data')
      .then((snapshot) => {
        return snapshot.val();
      }).then((customer) => {
        return stripe.customers.createSource(customer, {newPaymentSource});
      }).then((response) => {
        return change.after.ref.parent.set(response);
      }, (error) => {
        return change.after.ref.parent.child('error').set(userFacingMessage(error));
      }).then(() => {
        return reportError(error, {user: context.params.userId});
      });
 });