specify left join for filter associations if there was a previous left join

src/Bridge/Doctrine/Orm/Filter/AbstractFilter.php

@@ -13,6 +13,7 @@
 
 namespace ApiPlatform\Core\Bridge\Doctrine\Orm\Filter;
 
+use ApiPlatform\Core\Bridge\Doctrine\Orm\Util\QueryChecker;
 use ApiPlatform\Core\Bridge\Doctrine\Orm\Util\QueryNameGeneratorInterface;
 use ApiPlatform\Core\Exception\InvalidArgumentException;
 use ApiPlatform\Core\Util\RequestParser;
@@ -287,8 +288,14 @@ protected function addJoinOnce(QueryBuilder $queryBuilder, QueryNameGeneratorInt
 
         if (null === $join) {
             $associationAlias = $queryNameGenerator->generateJoinAlias($association);
-            $queryBuilder
-                ->join(sprintf('%s.%s', $alias, $association), $associationAlias);
+
+            if (true === QueryChecker::hasLeftJoin($queryBuilder)) {
+                $queryBuilder
+                    ->leftJoin(sprintf('%s.%s', $alias, $association), $associationAlias);
+            } else {
+                $queryBuilder
+                    ->innerJoin(sprintf('%s.%s', $alias, $association), $associationAlias);
+            }
         } else {
             $associationAlias = $join->getAlias();
         }

src/Bridge/Doctrine/Orm/Util/QueryChecker.php

@@ -15,6 +15,7 @@
 
 use Doctrine\Common\Persistence\ManagerRegistry;
 use Doctrine\ORM\Mapping\ClassMetadata;
+use Doctrine\ORM\Query\Expr\Join;
 use Doctrine\ORM\QueryBuilder;
 
 /**
@@ -155,4 +156,24 @@ public static function hasOrderByOnToManyJoin(QueryBuilder $queryBuilder, Manage
 
         return false;
     }
+
+    /**
+     * Determines whether the query builder already has a left join.
+     *
+     * @param QueryBuilder $queryBuilder
+     *
+     * @return bool
+     */
+    public static function hasLeftJoin(QueryBuilder $queryBuilder): bool
+    {
+        foreach ($queryBuilder->getDQLPart('join') as $dqlParts) {
+            foreach ($dqlParts as $dqlPart) {
+                if (Join::LEFT_JOIN === $dqlPart->getJoinType()) {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
 }

tests/Bridge/Doctrine/Orm/Filter/SearchFilterTest.php

@@ -767,4 +767,27 @@ public function testTripleJoin()
         $expected = strtolower(sprintf('SELECT o FROM %s o inner join o.relatedDummy relateddummy_a1 inner join relateddummy_a1.thirdLevel thirdLevel_a1 WHERE relateddummy_a1.symfony = :symfony_p1 and thirdLevel_a1.level = :level_p2', Dummy::class));
         $this->assertEquals($actual, $expected);
     }
+
+    public function testJoinLeft()
+    {
+        $request = Request::create('/api/dummies', 'GET', ['relatedDummy.symfony' => 'foo', 'relatedDummy.thirdLevel.level' => 'bar']);
+        $requestStack = new RequestStack();
+        $requestStack->push($request);
+        $queryBuilder = $this->repository->createQueryBuilder('o');
+        $queryBuilder->leftJoin('o.relatedDummy', 'relateddummy_a1');
+
+        $filter = new SearchFilter(
+            $this->managerRegistry,
+            $requestStack,
+            $this->iriConverter,
+            $this->propertyAccessor,
+            null,
+            ['relatedDummy.symfony' => null, 'relatedDummy.thirdLevel.level' => null]
+        );
+
+        $filter->apply($queryBuilder, new QueryNameGenerator(), $this->resourceClass, 'op');
+        $actual = strtolower($queryBuilder->getQuery()->getDQL());
+        $expected = strtolower(sprintf('SELECT o FROM %s o left join o.relatedDummy relateddummy_a1 left join relateddummy_a1.thirdLevel thirdLevel_a1 WHERE relateddummy_a1.symfony = :symfony_p1 and thirdLevel_a1.level = :level_p2', Dummy::class));
+        $this->assertEquals($actual, $expected);
+    }
 }