Failed getting entity with fetch eager property

src/UnitOfWork.php

@@ -3252,7 +3252,19 @@ private function eagerLoadCollections(array $collections, array $mapping): void
             foreach ($found as $targetValue) {
                 $sourceEntity = $targetProperty->getValue($targetValue);
 
-                $id     = $this->identifierFlattener->flattenIdentifier($class, $class->getIdentifierValues($sourceEntity));
+                if ($sourceEntity === null && isset($targetClass->associationMappings[$mappedBy]['joinColumns'])) {
+                    // case where the hydration $targetValue itself has not yet fully completed, for example
+                    // in case a bi-directional association is being hydrated and deferring eager loading is
+                    // not possible due to subclassing.
+                    $data = $this->getOriginalEntityData($targetValue);
+                    $id   = [];
+                    foreach ($targetClass->associationMappings[$mappedBy]['joinColumns'] as $joinColumn) {
+                        $id[] = $data[$joinColumn['name']];
+                    }
+                } else {
+                    $id = $this->identifierFlattener->flattenIdentifier($class, $class->getIdentifierValues($sourceEntity));
+                }
+
                 $idHash = implode(' ', $id);
 
                 if (isset($mapping['indexBy'])) {

tests/Tests/Models/AbstractFetchEager/AbstractRemoteControl.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\Models\AbstractFetchEager;
+
+use Doctrine\Common\Collections\ArrayCollection;
+use Doctrine\Common\Collections\Collection;
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity()
+ * @ORM\Table(name="abstract_fetch_eager_remote_control")
+ * @ORM\InheritanceType("SINGLE_TABLE")
+ * @ORM\DiscriminatorColumn(name="type", type="string")
+ * @ORM\DiscriminatorMap({"mobile"="MobileRemoteControl"})
+ */
+abstract class AbstractRemoteControl
+{
+    /**
+     * @ORM\Id
+     * @ORM\GeneratedValue
+     * @ORM\Column(type="integer")
+     *
+     * @var int
+     */
+    public $id;
+
+    /**
+     * /**
+     *
+     * @ORM\Column(type="string")
+     *
+     * @var string
+     */
+    public $name;
+
+    /**
+     * @ORM\OneToMany(targetEntity="User", mappedBy="remoteControl", fetch="EAGER")
+     *
+     * @var Collection<User>
+     */
+    public $users;
+
+    public function __construct(string $name)
+    {
+        $this->name  = $name;
+        $this->users = new ArrayCollection();
+    }
+}

tests/Tests/Models/AbstractFetchEager/MobileRemoteControl.php

@@ -0,0 +1,14 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\Models\AbstractFetchEager;
+
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity()
+ */
+class MobileRemoteControl extends AbstractRemoteControl
+{
+}

tests/Tests/Models/AbstractFetchEager/User.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\Models\AbstractFetchEager;
+
+use Doctrine\ORM\Mapping as ORM;
+
+/**
+ * @ORM\Entity()
+ * @ORM\Table(name="abstract_fetch_eager_user")
+ */
+class User
+{
+    /**
+     * @ORM\Id
+     * @ORM\GeneratedValue
+     * @ORM\Column(type="integer")
+     *
+     * @var int
+     */
+    public $id;
+
+    /**
+     * @ORM\ManyToOne(targetEntity="AbstractRemoteControl", inversedBy="users")
+     * @ORM\JoinColumn(nullable=false)
+     *
+     * @var AbstractRemoteControl
+     */
+    public $remoteControl;
+
+    public function __construct(AbstractRemoteControl $control)
+    {
+        $this->remoteControl = $control;
+    }
+}

tests/Tests/ORM/Functional/AbstractFetchEagerTest.php

@@ -0,0 +1,37 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\Tests\ORM\Functional;
+
+use Doctrine\Tests\Models\AbstractFetchEager\AbstractRemoteControl;
+use Doctrine\Tests\Models\AbstractFetchEager\MobileRemoteControl;
+use Doctrine\Tests\Models\AbstractFetchEager\User;
+use Doctrine\Tests\OrmFunctionalTestCase;
+
+final class AbstractFetchEagerTest extends OrmFunctionalTestCase
+{
+    public function testWithAbstractFetchEager(): void
+    {
+        $this->createSchemaForModels(
+            AbstractRemoteControl::class,
+            User::class
+        );
+
+        $control = new MobileRemoteControl('smart');
+        $user    = new User($control);
+
+        $entityManage = $this->getEntityManager();
+
+        $entityManage->persist($control);
+        $entityManage->persist($user);
+        $entityManage->flush();
+        $entityManage->clear();
+
+        $user = $entityManage->find(User::class, $user->id);
+
+        self::assertNotNull($user);
+        self::assertEquals('smart', $user->remoteControl->name);
+        self::assertTrue($user->remoteControl->users->contains($user));
+    }
+}