fix(core): sorting by multiple fields of the same relation

Author: alexzarbn

src/Drivers/Standard/QueryBuilder.php

@@ -395,7 +395,7 @@ function ($relationQuery) use ($relationField, $requestedSearchString, $caseSens
     /**
      * Apply sorting to the given query builder based on the "sort" query parameter.
      *
-     * @param Builder|Relation $query
+     * @param Builder $query
      * @param Request $request
      */
     public function applySortingToQuery($query, Request $request): void
@@ -431,8 +431,14 @@ public function applySortingToQuery($query, Request $request): void
                 );
                 $relationLocalKey = $this->relationsResolver->relationLocalKeyFromRelationInstance($relationInstance);
 
-                $query->leftJoin($relationTable, $relationForeignKey, '=', $relationLocalKey)
-                    ->orderBy("$relationTable.$relationField", $direction)
+                $requiresJoin = collect($query->toBase()->joins ?? [])
+                    ->where('table', $relationTable)->isEmpty();
+
+                if ($requiresJoin) {
+                    $query->leftJoin($relationTable, $relationForeignKey, '=', $relationLocalKey);
+                }
+
+                $query->orderBy("$relationTable.$relationField", $direction)
                     ->select($this->getQualifiedFieldName('*'));
             } else {
                 $query->orderBy($this->getQualifiedFieldName($sortableField), $direction);

tests/Feature/StandardIndexSortingOperationsTest.php

@@ -172,4 +172,29 @@ public function getting_a_list_of_resources_asc_sorted_by_relation_field(): void
             $this->makePaginator([$postA, $postB, $postC], 'posts/search')
         );
     }
+
+    /** @test */
+    public function getting_a_list_of_resources_asc_sorted_by_multiple_relation_fields(): void
+    {
+        $postC = factory(Post::class)->create(['user_id' => factory(User::class)->create(['name' => 'C'])->id])->fresh();
+        $postB = factory(Post::class)->create(['user_id' => factory(User::class)->create(['name' => 'B'])->id])->fresh();
+        $postA = factory(Post::class)->create(['user_id' => factory(User::class)->create(['name' => 'A'])->id])->fresh();
+
+        Gate::policy(Post::class, GreenPolicy::class);
+
+        $response = $this->post(
+            '/api/posts/search',
+            [
+                'sort' => [
+                    ['field' => 'user.name', 'direction' => 'asc'],
+                    ['field' => 'user.email', 'direction' => 'asc'],
+                ],
+            ]
+        );
+
+        $this->assertResourcesPaginated(
+            $response,
+            $this->makePaginator([$postA, $postB, $postC], 'posts/search')
+        );
+    }
 }

tests/Fixtures/app/Http/Controllers/PostsController.php

@@ -27,7 +27,7 @@ protected function beforeSave(Request $request, $entity)
 
     public function sortableBy(): array
     {
-        return ['title', 'user.name', 'meta->nested_field'];
+        return ['title', 'user.name', 'user.email', 'meta->nested_field'];
     }
 
     public function filterableBy(): array