[6.x] Backport for fix issue with polymorphic morphMaps with literal 0 (

#35364) (#35487)

* [8.x] Fix issue with polymorphic morphMaps with literal 0 (#35364)

* fix: use strict null check on morphTo type

* test: assert 0 relation_type is handled correctly

* Use setMethods for 6.x

* Update HasRelationships.php

Co-authored-by: Connor Tumbleson <iBotPeaches@users.noreply.github.com>

src/Illuminate/Database/Eloquent/Concerns/HasRelationships.php

@@ -233,7 +233,7 @@ public function morphTo($name = null, $type = null, $id = null, $ownerKey = null
         // If the type value is null it is probably safe to assume we're eager loading
         // the relationship. In this case we'll just pass in a dummy query where we
         // need to remove any eager loads that may already be defined on a model.
-        return empty($class = $this->{$type})
+        return is_null($class = $this->{$type}) || $class === ''
                     ? $this->morphEagerTo($name, $type, $id, $ownerKey)
                     : $this->morphInstanceTo($class, $name, $type, $id, $ownerKey);
     }

tests/Database/DatabaseEloquentMorphToTest.php

@@ -90,6 +90,16 @@ public function testMorphToWithArrayDefault()
         $this->assertSame('taylor', $result->username);
     }
 
+    public function testMorphToWithZeroMorphType()
+    {
+        $parent = $this->getMockBuilder(EloquentMorphToModelStub::class)->setMethods(['getAttribute', 'morphEagerTo', 'morphInstanceTo'])->getMock();
+        $parent->method('getAttribute')->with('relation_type')->willReturn(0);
+        $parent->expects($this->once())->method('morphInstanceTo');
+        $parent->expects($this->never())->method('morphEagerTo');
+
+        $parent->relation();
+    }
+
     public function testMorphToWithSpecifiedClassDefault()
     {
         $parent = new EloquentMorphToModelStub;