[16.x] Update SubscriptionItem quantity of a Single Price subscription.

src/Subscription.php

@@ -542,6 +542,12 @@ public function updateQuantity(int $quantity, ?string $price = null)
             'quantity' => $stripeSubscription->quantity,
         ])->save();
 
+        $singleSubscriptionItem = $this->items()->firstOrFail();
+
+        $singleSubscriptionItem->fill([
+            'quantity' => $stripeSubscription->quantity,
+        ])->save();
+
         $this->handlePaymentFailure($this);
 
         return $this;

tests/Feature/SubscriptionsTest.php

@@ -975,4 +975,25 @@ public function test_invoice_subscription_directly()
         $this->assertSame('paid', $invoice->status);
         $this->assertSame(2000, $invoice->total);
     }
+
+    public function test_updating_single_price_subscription_quantity_updates_the_quantity_of_the_subscription_item()
+    {
+        $user = $this->createCustomer('invoice_subscription_directly');
+
+        $subscription = $user->newSubscription('main', static::$priceId)
+            ->create('pm_card_visa');
+
+        $subscription->updateQuantity(2);
+
+        $this->assertDatabaseHas('subscriptions', [
+            'id' => $subscription->id,
+            'quantity' => 2,
+        ]);
+
+        $this->assertDatabaseHas('subscription_items', [
+            'subscription_id' => $subscription->id,
+            'stripe_price' => static::$priceId,
+            'quantity' => 2,
+        ]);
+    }
 }