Updates for code according to comments. Related updates for unit test.

Author: ivanko-dev

app/code/Magento/Quote/Api/CartManagementInterface.php

@@ -52,6 +52,9 @@ public function getCartForCustomer($customerId);
      * @param int $customerId The customer ID.
      * @param int $storeId
      * @return boolean
+     * @throws \Magento\Framework\Exception\LocalizedException
+     * @throws \Magento\Framework\Exception\StateException
+     * @throws \Magento\Framework\Exception\NoSuchEntityException
      */
     public function assignCustomer($cartId, $customerId, $storeId);
 

app/code/Magento/Quote/Model/QuoteManagement.php

@@ -305,13 +305,22 @@ public function assignCustomer($cartId, $customerId, $storeId)
         }
 
         if ($customerActiveQuote) {
-            /** Merge carts */
-            $quote->merge($customerActiveQuote);
-            $this->quoteRepository->delete($customerActiveQuote);
+            try {
+                /** Merge carts */
+                $quote->merge($customerActiveQuote);
+                $customerActiveQuote->setIsActive(0);
+                $this->quoteRepository->save($customerActiveQuote);
+            } catch (\Exception $e) {
+                $message = sprintf(
+                    "The customer can't be assigned to the cart. Error on cart merging: %s",
+                    $e->getMessage()
+                );
+                throw new StateException($message);
+            }
+
         }
         $quote->setCustomer($customer);
         $quote->setCustomerIsGuest(0);
-        $quote->setStoreId($storeId);
         $quote->setIsActive(1);
 
         /** @var \Magento\Quote\Model\QuoteIdMask $quoteIdMask */

app/code/Magento/Quote/Test/Unit/Model/QuoteManagementTest.php

@@ -510,12 +510,12 @@ public function testAssignCustomerWithActiveCart()
 
         $quoteMock = $this->createPartialMock(
             Quote::class,
-            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setStoreId', 'setIsActive', 'getIsActive', 'merge']
+            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setIsActive', 'getIsActive', 'merge']
         );
 
         $activeQuoteMock = $this->createPartialMock(
             Quote::class,
-            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setStoreId', 'setIsActive', 'getIsActive', 'merge']
+            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setIsActive', 'getIsActive', 'merge']
         );
 
         $customerMock = $this->createMock(CustomerInterface::class);
@@ -556,17 +556,17 @@ public function testAssignCustomerWithActiveCart()
             ->willReturn($activeQuoteMock);
 
         $quoteMock->expects($this->once())->method('merge')->with($activeQuoteMock)->willReturnSelf();
-        $this->quoteRepositoryMock->expects($this->once())->method('delete')->with($activeQuoteMock);
+        $activeQuoteMock->expects($this->once())->method('setIsActive')->with(0);
+        $this->quoteRepositoryMock->expects($this->atLeastOnce())->method('save')->with($activeQuoteMock);
 
         $quoteMock->expects($this->once())->method('setCustomer')->with($customerMock);
         $quoteMock->expects($this->once())->method('setCustomerIsGuest')->with(0);
-        $quoteMock->expects($this->once())->method('setStoreId')->with($storeId);
         $quoteMock->expects($this->once())->method('setIsActive')->with(1);
 
         $this->quoteIdMock->expects($this->once())->method('load')->with($cartId, 'quote_id')->willReturnSelf();
         $this->quoteIdMock->expects($this->once())->method('getId')->willReturn(10);
         $this->quoteIdMock->expects($this->once())->method('delete');
-        $this->quoteRepositoryMock->expects($this->once())->method('save')->with($quoteMock);
+        $this->quoteRepositoryMock->expects($this->atLeastOnce())->method('save')->with($quoteMock);
 
         $this->model->assignCustomer($cartId, $customerId, $storeId);
     }
@@ -585,7 +585,7 @@ public function testAssignCustomer()
 
         $quoteMock = $this->createPartialMock(
             Quote::class,
-            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setStoreId', 'setIsActive', 'getIsActive', 'merge']
+            ['getCustomerId', 'setCustomer', 'setCustomerIsGuest', 'setIsActive', 'getIsActive', 'merge']
         );
 
         $customerMock = $this->createMock(CustomerInterface::class);
@@ -629,11 +629,9 @@ public function testAssignCustomer()
 
         $this->assertEquals(false, $activeQuoteMock);
         $quoteMock->expects($this->never())->method('merge');
-        $this->quoteRepositoryMock->expects($this->never())->method('delete');
 
         $quoteMock->expects($this->once())->method('setCustomer')->with($customerMock);
         $quoteMock->expects($this->once())->method('setCustomerIsGuest')->with(0);
-        $quoteMock->expects($this->once())->method('setStoreId')->with($storeId);
         $quoteMock->expects($this->once())->method('setIsActive')->with(1);
 
         $this->quoteIdMock->expects($this->once())->method('load')->with($cartId, 'quote_id')->willReturnSelf();