Implemented error handling for unique key violations

src/main/java/com/nisum/ascend/inventory/controller/ProductController.java

 package com.nisum.ascend.inventory.controller;
 
 import com.nisum.ascend.inventory.dto.ProductDto;
+import com.nisum.ascend.inventory.exception.ResourceAlreadyExistsException;
 import com.nisum.ascend.inventory.model.Product;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.http.HttpStatus;
@@ -9,6 +10,7 @@ import org.springframework.web.bind.annotation.PostMapping;
 import org.springframework.web.bind.annotation.RequestBody;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.reactive.function.server.ServerResponse;
 import reactor.core.publisher.Mono;
 import com.nisum.ascend.inventory.service.ProductService;
 
@@ -19,8 +21,7 @@ public class ProductController {
     ProductService productService;
 
     @PostMapping("")
-    public ResponseEntity<Mono<ProductDto>> postProduct(@RequestBody Product product) {
-        Mono<ProductDto> newProduct = productService.createProduct(product);
-        return new ResponseEntity<>(newProduct, HttpStatus.ACCEPTED);
+    public Mono<ProductDto> postProduct(@RequestBody Product product) {
+        return productService.createProduct(product).onErrorMap(throwable -> new ResourceAlreadyExistsException());
     }
 }
\ No newline at end of file

src/main/java/com/nisum/ascend/inventory/dto/ProductDto.java

@@ -18,6 +18,7 @@ public class ProductDto {
     private int stock;
     private PromotionDto promotion;
 
+    public ProductDto() {}
 
     public static ProductDto generateDtoFromProduct(Product product) {
         // TODO: Fetch the product's promotion from the Promotions com.nisum.ascend.inventory.service

src/main/java/com/nisum/ascend/inventory/exception/ResourceAlreadyExistsException.java

+package com.nisum.ascend.inventory.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(value = HttpStatus.NOT_ACCEPTABLE)
+public class ResourceAlreadyExistsException extends RuntimeException {
+
+    public ResourceAlreadyExistsException() {
+        super("A resource with the provided SKU already exists");
+    }
+}

src/main/java/com/nisum/ascend/inventory/exception/RestWebExceptionHandler.java

+package com.nisum.ascend.inventory.exception;
+
+import com.mongodb.DuplicateKeyException;
+import org.springframework.core.annotation.Order;
+import org.springframework.http.HttpStatus;
+import org.springframework.stereotype.Component;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebExceptionHandler;
+import reactor.core.publisher.Mono;
+
+@Component
+@Order(-2)
+class RestWebExceptionHandler implements WebExceptionHandler {
+
+    @Override
+    public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
+        if (ex instanceof ResourceAlreadyExistsException) {
+            exchange.getResponse().setStatusCode(HttpStatus.NOT_ACCEPTABLE);
+
+            // marks the response as complete and forbids writing to it
+            return exchange.getResponse().setComplete();
+        }
+        return Mono.error(ex);
+    }
+}

src/main/java/com/nisum/ascend/inventory/service/ProductService.java

 package com.nisum.ascend.inventory.service;
 
 import com.nisum.ascend.inventory.dto.ProductDto;
+import com.nisum.ascend.inventory.exception.ResourceAlreadyExistsException;
 import com.nisum.ascend.inventory.model.Product;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Service;
+import org.springframework.web.reactive.function.server.ServerResponse;
 import reactor.core.publisher.Mono;
 import com.nisum.ascend.inventory.repository.ProductRepository;
 
@@ -13,9 +15,7 @@ public class ProductService {
     ProductRepository productRepository;
 
     public Mono<ProductDto> createProduct(Product product) {
-
         return productRepository.save(product)
-                .map(ProductDto::generateDtoFromProduct)
-
+                .map(ProductDto::generateDtoFromProduct);
     }
 }