feat(medusa): Add product isolation FF to update cart (#5168)

**What**
Update cart must use the remote query when being in product isolation mode.

This pr is a first iteration that aims to quickly make compatible the end point in isolation. In a future iteration, this end point will be migrated to a workflow

Co-authored-by: Oli Juhl <59018053+olivermrbl@users.noreply.github.com>

packages/medusa/src/api/routes/store/carts/update-cart.ts

@@ -15,6 +15,7 @@ import { AddressPayload } from "../../../../types/common"
 import { FeatureFlagDecorators } from "../../../../utils/feature-flag-decorators"
 import { IsType } from "../../../../utils/validators/is-type"
 import { cleanResponseData } from "../../../../utils/clean-response-data"
+import IsolateProductDomainFeatureFlag from "../../../../loaders/feature-flags/isolate-product-domain"
 
 /**
  * @oas [post] /store/carts/{id}
@@ -75,14 +76,22 @@ export default async (req, res) => {
   const validated = req.validatedBody as StorePostCartsCartReq
 
   const cartService: CartService = req.scope.resolve("cartService")
+  const featureFlagRouter = req.scope.resolve("featureFlagRouter")
   const manager: EntityManager = req.scope.resolve("manager")
 
   if (req.user?.customer_id) {
     validated.customer_id = req.user.customer_id
   }
 
+  let cart
+  if (featureFlagRouter.isFeatureEnabled(IsolateProductDomainFeatureFlag.key)) {
+    cart = await retrieveCartWithIsolatedProductModule(req, id)
+  }
+
   await manager.transaction(async (transactionManager) => {
-    await cartService.withTransaction(transactionManager).update(id, validated)
+    await cartService
+      .withTransaction(transactionManager)
+      .update(cart ?? id, validated)
 
     const updated = await cartService
       .withTransaction(transactionManager)
@@ -101,9 +110,60 @@ export default async (req, res) => {
     select: defaultStoreCartFields,
     relations: defaultStoreCartRelations,
   })
+
   res.json({ cart: cleanResponseData(data, []) })
 }
 
+async function retrieveCartWithIsolatedProductModule(req, id: string) {
+  const cartService = req.scope.resolve("cartService")
+  const remoteQuery = req.scope.resolve("remoteQuery")
+
+  const relations = [
+    "items",
+    "shipping_methods",
+    "shipping_methods.shipping_option",
+    "shipping_address",
+    "billing_address",
+    "gift_cards",
+    "customer",
+    "region",
+    "payment_sessions",
+    "region.countries",
+    "discounts",
+    "discounts.rule",
+  ]
+
+  const cart = await cartService.retrieve(id, {
+    relations,
+  })
+
+  const products = await remoteQuery({
+    products: {
+      __args: {
+        id: cart.items.map((i) => i.product_id),
+      },
+      fields: ["id"],
+      variants: {
+        fields: ["id"],
+      },
+    },
+  })
+
+  const variantsMap = new Map(
+    products.flatMap((p) => p.variants).map((v) => [v.id, v])
+  )
+
+  cart.items.forEach((item) => {
+    if (!item.variant_id) {
+      return
+    }
+
+    item.variant = variantsMap.get(item.variant_id)
+  })
+
+  return cart
+}
+
 class GiftCard {
   @IsString()
   code: string