Fix for GitHub issue #14035. Explode a string of category filter. Change the default condition type to in.

Author: kamilszewczyk

app/code/Magento/Catalog/Model/Api/SearchCriteria/CollectionProcessor/FilterProcessor/ProductCategoryFilter.php

@@ -21,8 +21,14 @@ class ProductCategoryFilter implements CustomFilterInterface
      */
     public function apply(Filter $filter, AbstractDb $collection)
     {
-        $conditionType = $filter->getConditionType() ? $filter->getConditionType() : 'eq';
-        $categoryFilter = [$conditionType => [$filter->getValue()]];
+        $value = $filter->getValue();
+        $conditionType = $filter->getConditionType() ? $filter->getConditionType() : 'in';
+        if (($conditionType == 'in' || $conditionType == 'nin') && is_string($value)) {
+            $value = explode(',', $value);
+        } else {
+            $value = [$value];
+        }
+        $categoryFilter = [$conditionType => $value];
 
         /** @var Collection $collection */
         $collection->addCategoriesFilter($categoryFilter);

app/code/Magento/Catalog/Test/Unit/Model/Api/SearchCriteria/CollectionProcessor/FilterProcessor/ProductCategoryFilterTest.php

@@ -66,7 +66,7 @@ public function testApplyWithoutCondition()
 
         $collectionMock->expects($this->once())
             ->method('addCategoriesFilter')
-            ->with(['eq' => ['value']]);
+            ->with(['in' => ['value']]);
 
         $this->assertTrue($this->model->apply($filterMock, $collectionMock));
     }