[API][ProductTaxon] Add unique combination validation

features/product/managing_products/modifying_taxons_assigned_to_an_existing_product.feature

@@ -21,3 +21,8 @@ Feature: Modifying taxons assigned to an exisiting product
     Scenario: Adding taxons to product
         When I add "Clothes" taxon to the "Shirt" product
         Then the product "Shirt" should have the "Clothes" taxon
+
+    @api @no-ui
+    Scenario: Being prevented from adding the same taxon twice
+        When I try to add "Clothes" taxon to the "T-Shirt" product
+        Then I should be notified that this taxon is already assigned to this product

features/product/managing_products/product_validation.feature

@@ -114,14 +114,6 @@ Feature: Products validation
         Then I should be notified that name is required
         And this product should still be named "Dice Brewing"
 
-    @ui @api
-    Scenario: Not seeing validation error for duplicated code if product code has not been changed
-        Given the store has a "Dice Brewing" product
-        When I want to modify this product
-        And I remove its name from "English (United States)" translation
-        And I try to save my changes
-        Then this product should still be named "Dice Brewing"
-
     @ui
     Scenario: Trying to assign new channel to an existing configurable product without specifying its all variant prices for this channel
         Given the store has a "7 Wonders" configurable product

src/Sylius/Behat/Context/Api/Admin/ManagingProductTaxonsContext.php

@@ -19,6 +19,7 @@
 use Sylius\Behat\Context\Api\Resources;
 use Sylius\Component\Core\Model\ProductInterface;
 use Sylius\Component\Core\Model\TaxonInterface;
+use Webmozart\Assert\Assert;
 
 final class ManagingProductTaxonsContext implements Context
 {
@@ -38,7 +39,7 @@ public function iChangeThatTheProductBelongsToTheTaxon(ProductInterface $product
 
     /**
      * @When I assign the :taxon taxon to the :product product
-     * @When I add :taxon taxon to the :product product
+     * @When I (try to) add :taxon taxon to the :product product
      */
     public function iAddTaxonToTheProduct(TaxonInterface $taxon, ProductInterface $product): void
     {
@@ -47,4 +48,15 @@ public function iAddTaxonToTheProduct(TaxonInterface $taxon, ProductInterface $p
         $this->client->addRequestData('product', $this->iriConverter->getIriFromItem($product));
         $this->client->create();
     }
+
+    /**
+     * @Then I should be notified that this taxon is already assigned to this product
+     */
+    public function iShouldBeNotifiedThatThisTaxonIsAlreadyAssignedToThisProduct(): void
+    {
+        Assert::contains(
+            $this->client->getLastResponse()->getContent(),
+            'This taxon is already assigned to this product.',
+        );
+    }
 }

src/Sylius/Bundle/CoreBundle/Resources/config/validation/ProductTaxon.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+
+<!--
+
+ This file is part of the Sylius package.
+
+ (c) Paweł Jędrzejewski
+
+ For the full copyright and license information, please view the LICENSE
+ file that was distributed with this source code.
+
+-->
+
+<constraint-mapping xmlns="http://symfony.com/schema/dic/constraint-mapping"
+                    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+                    xsi:schemaLocation="http://symfony.com/schema/dic/constraint-mapping https://symfony.com/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">
+    <class name="Sylius\Component\Core\Model\ProductTaxon">
+        <constraint name="Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity">
+            <option name="fields">
+                <value>product</value>
+                <value>taxon</value>
+            </option>
+            <option name="message">sylius.product_taxon.unique</option>
+            <option name="groups">sylius</option>
+        </constraint>
+    </class>
+</constraint-mapping>

src/Sylius/Bundle/CoreBundle/Resources/translations/validators.en.yml

@@ -88,6 +88,8 @@ sylius:
         file:
             max_size: The image is too big - {{ size }}{{ suffix }}. Maximum allowed size is {{ limit }}{{ suffix }}.
             upload_ini_size: The image is too big. Maximum allowed size is {{ limit }}{{ suffix }}.
+    product_taxon:
+        unique: This taxon is already assigned to this product.
     product_variant:
         onHand:
             min: On hand must be greater than {{ limit }}.