Remove stdClass support, enable pure entity trees

Drop stdClass entities: all hydration now produces typed classes.
Simplify extractColumns to a one-liner delegating to
EntityFactory::extractColumns() + filterColumns(). Simplify
getRelatedEntity keeping the FK-field fallback for backward
compatibility with OtherEntity-style setters.

Entity stubs use pure entity trees: relation properties ($post,
$author) replace FK scalars. OtherEntity\Post marks its derived
$author as #[NotPersistable] since it keeps explicit $author_id.

Remove 5 redundant tests (identical duplicate + fetch/fetchAll
pairs). Add coverage for LEFT JOIN null-child, non-existent row
fetch, pure entity tree FK derivation on persist.

Author: alganet

src/Mapper.php

@@ -18,7 +18,6 @@
 use SplObjectStorage;
 use Throwable;
 
-use function array_key_exists;
 use function array_keys;
 use function array_merge;
 use function array_push;
@@ -287,33 +286,10 @@ private function generateQuery(Collection $collection): Sql
     /** @return array<string, mixed> */
     private function extractColumns(object $entity, Collection $collection): array
     {
-        $primaryName = $this->style->identifier($collection->name);
-        $cols = $this->entityFactory->extractProperties($entity);
-
-        foreach ($cols as $key => $c) {
-            if (is_object($c) && $this->style->isRelationProperty($key)) {
-                unset($cols[$key]);
-
-                continue;
-            }
-
-            if (is_object($c)) {
-                $cols[$key] = $this->entityFactory->get($c, $primaryName);
-
-                continue;
-            }
-
-            if (
-                !$this->style->isRelationProperty($key)
-                || !array_key_exists($this->style->remoteIdentifier($key), $cols)
-            ) {
-                continue;
-            }
-
-            unset($cols[$key]);
-        }
-
-        return $this->filterColumns($cols, $collection);
+        return $this->filterColumns(
+            $this->entityFactory->extractColumns($entity),
+            $collection,
+        );
     }
 
     /** @param array<string, Collection> $collections */

tests/Hydrators/FlatNumTest.php

@@ -11,7 +11,7 @@
 use Respect\Data\Collections\Collection;
 use Respect\Data\Collections\Typed;
 use Respect\Data\EntityFactory;
-use stdClass;
+use Respect\Relational\Bug;
 
 #[CoversClass(FlatNum::class)]
 class FlatNumTest extends TestCase
@@ -28,7 +28,7 @@ protected function setUp(): void
         $this->pdo->exec('CREATE TABLE post (id INTEGER PRIMARY KEY, title TEXT, author_id INTEGER)');
         $this->pdo->exec("INSERT INTO author VALUES (1, 'Alice')");
         $this->pdo->exec("INSERT INTO post VALUES (10, 'Hello', 1)");
-        $this->factory = new EntityFactory();
+        $this->factory = new EntityFactory(entityNamespace: 'Respect\\Relational\\');
     }
 
     #[Test]
@@ -83,19 +83,18 @@ public function hydrateReturnsFalseForEmptyResult(): void
     public function hydrateResolvesTypedEntity(): void
     {
         $this->pdo->exec('CREATE TABLE issue (id INTEGER PRIMARY KEY, title TEXT, type TEXT)');
-        $this->pdo->exec("INSERT INTO issue VALUES (1, 'Bug Report', 'stdClass')");
+        $this->pdo->exec("INSERT INTO issue VALUES (1, 'Bug Report', 'Bug')");
 
         $stmt = $this->pdo->prepare('SELECT id, title, type FROM issue');
         $stmt->execute();
         $row = $stmt->fetch(PDO::FETCH_NUM);
 
-        $factory = new EntityFactory(entityNamespace: 'Respect\Relational\Hydrators\\');
         $hydrator = new FlatNum($stmt);
         $collection = Typed::issue('type');
-        $result = $hydrator->hydrate($row, $collection, $factory);
+        $result = $hydrator->hydrate($row, $collection, $this->factory);
 
         $this->assertNotFalse($result);
         $result->rewind();
-        $this->assertInstanceOf(stdClass::class, $result->current());
+        $this->assertInstanceOf(Bug::class, $result->current());
     }
 }