[MorphTo] Preserve string "0" keys when gathering eager load IDs

src/Illuminate/Database/Eloquent/Relations/MorphTo.php

@@ -185,7 +185,9 @@ protected function gatherKeysByType($type, $keyType)
             ? array_keys($this->dictionary[$type])
             : array_map(function ($modelId) {
                 return (string) $modelId;
-            }, array_filter(array_keys($this->dictionary[$type])));
+            }, array_filter(array_keys($this->dictionary[$type]), static function ($modelId) {
+                return $modelId !== '';
+            }));
     }
 
     /**

tests/Database/DatabaseEloquentMorphToTest.php

@@ -8,6 +8,7 @@
 use Illuminate\Tests\Database\stubs\TestEnum;
 use Mockery as m;
 use PHPUnit\Framework\TestCase;
+use ReflectionMethod;
 
 class DatabaseEloquentMorphToTest extends TestCase
 {
@@ -68,6 +69,21 @@ public function __toString()
         ], $dictionary);
     }
 
+    public function testGatherKeysByTypeKeepsStringZeroForStringKeyTypes()
+    {
+        $relation = $this->getRelation();
+
+        $relation->addEagerConstraints([
+            (object) ['morph_type' => 'morph_type_1', 'foreign_key' => '0'],
+            (object) ['morph_type' => 'morph_type_1', 'foreign_key' => ''],
+        ]);
+
+        $method = new ReflectionMethod(MorphTo::class, 'gatherKeysByType');
+        $method->setAccessible(true);
+
+        $this->assertSame(['0'], $method->invoke($relation, 'morph_type_1', 'string'));
+    }
+
     public function testMorphToWithDefault()
     {
         $relation = $this->getRelation()->withDefault();