Try to add a reproducer for #11861

tests/Tests/ORM/Functional/Ticket/GH11861Test.php

@@ -0,0 +1,106 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\ORM\Exception\ORMException;
+use Doctrine\ORM\Mapping as ORM;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+class GH11861Test extends OrmFunctionalTestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->setUpEntitySchema([
+            GH11861BaseEntity::class,
+            GH11861ChildEntityA::class,
+        ]);
+    }
+
+    /**
+     * @throws ORMException
+     */
+    public function testEntityMapCollision(): void
+    {
+        $externalId = 'external-123'; // Predefined/external ID
+
+        // Create and persist entity
+        $entity = new GH11861ChildEntityA();
+        $entity->setId($externalId);
+        $entity->setFieldA('test');
+        $this->_em->persist($entity);
+        $this->_em->flush();
+        $this->_em->clear();
+
+        // Fetch via parent class
+        $baseEntity = $this->_em->find(GH11861BaseEntity::class, $externalId); // Returns ChildEntityA
+
+        // Fetch via child class
+        $childEntity = $this->_em->find(GH11861ChildEntityA::class, $externalId); // New instance created
+
+        // Check if they’re the same
+        self::assertSame($baseEntity, $childEntity);
+
+        // Trigger collision
+        $this->_em->flush(); // Throws identity collision error
+    }
+}
+
+/**
+ * @ORM\Entity
+ * @ORM\InheritanceType("JOINED")
+ * @ORM\DiscriminatorColumn(name="type", type="string")
+ * @ORM\DiscriminatorMap({"a" = "GH11861ChildEntityA"})
+ */
+#[ORM\Entity]
+#[ORM\InheritanceType('JOINED')]
+#[ORM\DiscriminatorColumn(name: 'type', type: 'string')]
+#[ORM\DiscriminatorMap(['a' => GH11861ChildEntityA::class])]
+class GH11861BaseEntity
+{
+    /**
+     * @ORM\Id
+     * @ORM\Column(type="string") // Predefined/external ID
+     */
+    #[ORM\Id]
+    #[ORM\Column(type: 'string')] // Predefined/external ID
+    protected $id;
+
+    public function setId($id)
+    {
+        $this->id = $id;
+    }
+
+    public function getId()
+    {
+        return $this->id;
+    }
+}
+
+// Child entity
+
+/**
+ * @ORM\Entity
+ */
+#[ORM\Entity]
+class GH11861ChildEntityA extends GH11861BaseEntity
+{
+    /**
+     * @ORM\Column(type="string")
+     */
+    #[ORM\Column(type: 'string')]
+    private $fieldA;
+
+    public function setFieldA($value)
+    {
+        $this->fieldA = $value;
+    }
+
+    public function getFieldA()
+    {
+        return $this->fieldA;
+    }
+}