Fix incorrect invoice lines on subscription plan upgrade

[adrienverge]

InvoiceItem: Include 'plan' and 'subscription' info if available

That's what real Stripe does, e.g. on a plan upgrade:

[
  {
    "id": "il_1GbU4aKxWWXl12QBOzUyETIM",
    "type": "invoiceitem",
    "subscription_item": "si_H9nhRadSKnHfBu",
    "invoice_item": "ii_1GbU4bKxWWXl12QBhGX2ce3u",
    "amount": -1000,
    "description": "Unused time on Plan after 24 Apr 2020",
    "plan": { "id": "monthly-plan", ... },
    "proration": true
  },
  {
    "id": "il_1GbU4bKxWWXl12QCMJnxK6Ws",
    "type": "subscription",
    "subscription_item": "si_H9nhRadSKnHfBu",
    "amount": 9900,
    "currency": "eur",
    "description": "1 × Plan (at €99.00 / year)",
    "plan": { "id": "yearly-plan", ... },
    "proration": false
  }
]

---

Invoice: Create SubscriptionItems (not InvoiceItems) for upcoming

This fixes a bug when a subscription is upgraded to another plan with a different billing cycle (e.g. monthly to yearly). In such a case, the invoice is generated when the user creates the invoice then calls API /pay, but in this case the invoice is made from Invoice._get_next_invoice(), not from Subscription._create_invoice().

Subsciption items need to be created as SubscriptionItem, not InvoiceItem (since commit a80dda2 "feat: Support invoice line items (il_) from Stripe API 2019-12-03").

[H--o-l]

Hey @adrienverge !

I propose to add the following unit test inside one of the two commit (feel free to change or improve it!):

--- a/test.sh
+++ b/test.sh
@@ -469,6 +469,29 @@ curl -sSf -u $SK: $HOST/v1/subscriptions/$sub \

 curl -sSf -u $SK: $HOST/v1/invoices?customer=$cus

+cus=$(curl -sSf -u $SK: $HOST/v1/customers \
+           -d description='This customer will switch from a yearly to another yearly plan' \
+           -d email=switch@bar.com \
+      | grep -oE 'cus_\w+' | head -n 1)
+
+curl -sSf -u $SK: $HOST/v1/customers/$cus/sources \
+     -d source=$tok
+
+sub=$(curl -sSf -u $SK: $HOST/v1/subscriptions \
+           -d customer=$cus \
+           -d items[0][plan]=basique-annuel)
+sub_id=$(echo "$sub" | grep -oE 'sub_\w+' | head -n 1)
+sub_item_id=$(echo "$sub" | grep -oE 'si_\w+' | head -n 1)
+
+sub=$(curl -sSf -u $SK: $HOST/v1/subscriptions/$sub_id \
+           -d items[0][plan]=pro-annuel \
+           -d items[0][id]=$sub_item_id)
+
+in=$(curl -sSf -u $SK: $HOST/v1/invoices \
+          -d customer=$cus)
+echo $in | grep "Abonnement PRO (annuel)"
+echo $in | grep "Abonnement basique (annuel)"
+
 cus=$(curl -sSf -u $SK: $HOST/v1/customers \
            -d email=john.malkovich@example.com \
       | grep -oE 'cus_\w+' | head -n 1)

I checked that before the PR it fails, and on it, it works.

[adrienverge]

I propose to add the following unit test inside one of the two commit (feel free to change or improve it!):

So nice!!!

I took it as is, and just changed this for a better display (otherwise it's very hard to read and debug), otherwise, thanks :ok_hand:

-echo $in | grep "Abonnement PRO (annuel)"
-echo $in | grep "Abonnement basique (annuel)"
+grep -q "Abonnement PRO (annuel)" <<<"$in"
+grep -q "Abonnement basique (annuel)" <<<"$in"