fix: Handle region updates on cart (1/n) (#9369)

**What**

On cart creation:
- If region only has one country -> create cart with country code

On cart updates:
- If shipping address country code is provided in input ->
  - If cart region doesn't include that country -> throw
  - If cart includes the country -> update shipping address
- If region is provided in input and is different from the one currently on the cart -> 
  - If there is a shipping address on the cart -> clear the address
    - If the region only has one country -> set country code of address
  - If there is not a shipping address on the cart ->
    - If the region only has one country -> set country code of address


Closes CC-545

packages/core/core-flows/src/cart/steps/find-one-or-any-region.ts

@@ -18,7 +18,9 @@ export const findOneOrAnyRegionStep = createStep(
 
     if (data.regionId) {
       try {
-        const region = await service.retrieveRegion(data.regionId)
+        const region = await service.retrieveRegion(data.regionId, {
+          relations: ["countries"],
+        })
         return new StepResponse(region)
       } catch (error) {
         return new StepResponse(null)
@@ -31,9 +33,12 @@ export const findOneOrAnyRegionStep = createStep(
       throw new MedusaError(MedusaError.Types.NOT_FOUND, "Store not found")
     }
 
-    const [region] = await service.listRegions({
-      id: store.default_region_id,
-    })
+    const [region] = await service.listRegions(
+      {
+        id: store.default_region_id,
+      },
+      { relations: ["countries"] }
+    )
 
     if (!region) {
       return new StepResponse(null)

packages/core/core-flows/src/cart/workflows/create-carts.ts

@@ -27,9 +27,6 @@ import { refreshPaymentCollectionForCartWorkflow } from "./refresh-payment-colle
 import { updateCartPromotionsWorkflow } from "./update-cart-promotions"
 import { updateTaxLinesWorkflow } from "./update-tax-lines"
 
-// TODO: The createCartWorkflow are missing the following steps:
-// - Refresh/delete shipping methods (fulfillment module)
-
 export const createCartWorkflowId = "create-cart"
 /**
  * This workflow creates a cart.
@@ -119,6 +116,13 @@ export const createCartWorkflow = createWorkflow(
           data_.sales_channel_id = data.salesChannel.id
         }
 
+        // If there is only one country in the region, we prepare a shipping address with that country's code.
+        if (!data.input.shipping_address && data.region.countries.length === 1) {
+          data_.shipping_address = {
+            country_code: data.region.countries[0].iso_2,
+          }
+        }
+
         return data_
       }
     )

packages/core/core-flows/src/cart/workflows/update-cart.ts

@@ -2,11 +2,7 @@ import {
   AdditionalData,
   UpdateCartWorkflowInputDTO,
 } from "@medusajs/framework/types"
-import {
-  MedusaError,
-  PromotionActions,
-  isPresent,
-} from "@medusajs/framework/utils"
+import { MedusaError, PromotionActions } from "@medusajs/framework/utils"
 import {
   WorkflowData,
   WorkflowResponse,
@@ -14,10 +10,10 @@ import {
   createWorkflow,
   parallelize,
   transform,
+  when,
 } from "@medusajs/framework/workflows-sdk"
 import { useRemoteQueryStep } from "../../common"
 import {
-  findOneOrAnyRegionStep,
   findOrCreateCustomerStep,
   findSalesChannelStep,
   refreshCartShippingMethodsStep,
@@ -35,35 +31,85 @@ export const updateCartWorkflowId = "update-cart"
 export const updateCartWorkflow = createWorkflow(
   updateCartWorkflowId,
   (input: WorkflowData<UpdateCartWorkflowInputDTO & AdditionalData>) => {
-    const [salesChannel, region, customerData] = parallelize(
+    const cartToUpdate = useRemoteQueryStep({
+      entry_point: "cart",
+      variables: { id: input.id },
+      fields: ["id", "shipping_address.*", "region.*", "region.countries.*"],
+      list: false,
+      throw_if_key_not_found: true,
+    }).config({ name: "get-cart" })
+
+    const [salesChannel, customerData] = parallelize(
       findSalesChannelStep({
         salesChannelId: input.sales_channel_id,
       }),
-      findOneOrAnyRegionStep({
-        regionId: input.region_id,
-      }),
       findOrCreateCustomerStep({
         customerId: input.customer_id,
         email: input.email,
       })
     )
 
+    const newRegion = when({ input }, (data) => {
+      return !!data.input.region_id
+    }).then(() => {
+      return useRemoteQueryStep({
+        entry_point: "region",
+        variables: { id: input.region_id },
+        fields: ["id", "countries.*", "currency_code", "name"],
+        list: false,
+        throw_if_key_not_found: true,
+      }).config({ name: "get-region" })
+    })
+
+    const region = transform({ cartToUpdate, newRegion }, (data) => {
+      return data.newRegion ?? data.cartToUpdate.region
+    })
+
     const cartInput = transform(
-      { input, region, customerData, salesChannel },
+      { input, region, customerData, salesChannel, cartToUpdate },
       (data) => {
         const { promo_codes, ...updateCartData } = data.input
-        const data_ = { ...updateCartData }
 
-        if (isPresent(updateCartData.region_id)) {
-          if (!data.region) {
+        const data_ = {
+          ...updateCartData,
+          currency_code: data.region?.currency_code,
+          region_id: data.region?.id, // This is either the region from the input or the region from the cart or null
+        }
+
+        // When the region is updated, we do a few things:
+        // - We need to make sure the provided shipping address country code is in the new region
+        // - We clear the shipping address if the new region has more than one country
+        const regionIsNew = data.region?.id !== data.cartToUpdate.region?.id
+        const shippingAddress = data.input.shipping_address
+
+        if (shippingAddress?.country_code) {
+          const country = data.region.countries.find(
+            (c) => c.iso_2 === shippingAddress.country_code
+          )
+
+          if (!country) {
             throw new MedusaError(
-              MedusaError.Types.NOT_FOUND,
-              "Region not found"
+              MedusaError.Types.INVALID_DATA,
+              `Country with code ${shippingAddress.country_code} is not within region ${data.region.name}`
             )
           }
 
-          data_.currency_code = data.region.currency_code
-          data_.region_id = data.region.id
+          data_.shipping_address = {
+            ...shippingAddress,
+            country_code: country.iso_2,
+          }
+        }
+
+        if (regionIsNew) {
+          if (data.region.countries.length === 1) {
+            data_.shipping_address = {
+              country_code: data.region.countries[0].iso_2,
+            }
+          }
+
+          if (!data_.shipping_address?.country_code) {
+            data_.shipping_address = null
+          }
         }
 
         if (