Add failing tests for #6531

Tests are based on examples from "Composite and Foreign Keys as Primary Key" tutorial:
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/tutorials/composite-primary-keys.html

tests/Doctrine/Tests/ORM/Functional/Ticket/GH6531Test.php

@@ -0,0 +1,190 @@
+<?php
+
+namespace Doctrine\Tests\ORM\Functional\Ticket;
+
+use Doctrine\Common\Collections\ArrayCollection;
+
+final class GH6531Test extends \Doctrine\Tests\OrmFunctionalTestCase
+{
+    protected function setUp() : void
+    {
+        parent::setup();
+
+        $this->setUpEntitySchema(
+            [
+                GH6531User::class,
+                GH6531Address::class,
+                GH6531Article::class,
+                GH6531ArticleAttribute::class,
+                GH6531Order::class,
+                GH6531OrderItem::class,
+                GH6531Product::class,
+            ]
+        );
+    }
+
+    /**
+     * @group 6531
+     */
+    public function testSimpleDerivedIdentity() : void
+    {
+        $user          = new GH6531User();
+        $address       = new GH6531Address();
+        $address->user = $user;
+
+        $this->_em->persist($user);
+        $this->_em->persist($address);
+        $this->_em->flush();
+
+        self::assertSame($user, $this->_em->find(GH6531User::class, $user->id));
+        self::assertSame($address, $this->_em->find(GH6531Address::class, $user));
+    }
+
+    /**
+     * @group 6531
+     */
+    public function testDynamicAttributes() : void
+    {
+        $article = new GH6531Article();
+        $article->addAttribute('name', 'value');
+
+        $this->_em->persist($article);
+        $this->_em->flush();
+
+        self::assertSame(
+            $article->attributes['name'],
+            $this->_em->find(GH6531ArticleAttribute::class, ['article' => $article, 'attribute' => 'name'])
+        );
+    }
+
+    /**
+     * @group 6531
+     */
+    public function testJoinTableWithMetadata() : void
+    {
+        $product = new GH6531Product();
+        $this->_em->persist($product);
+        $this->_em->flush();
+
+        $order = new GH6531Order();
+        $order->addItem($product, 2);
+
+        $this->_em->persist($order);
+        $this->_em->flush();
+
+        self::assertSame(
+            $order->items->first(),
+            $this->_em->find(GH6531OrderItem::class, ['product' => $product, 'order' => $order])
+        );
+    }
+}
+
+/**
+ * @Entity
+ */
+class GH6531User
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+}
+
+/**
+ * @Entity
+ */
+class GH6531Address
+{
+    /** @Id @OneToOne(targetEntity=GH6531User::class) */
+    public $user;
+}
+
+/**
+ * @Entity
+ */
+class GH6531Article
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+
+    /** @OneToMany(targetEntity=GH6531ArticleAttribute::class, mappedBy="article", cascade={"ALL"}, indexBy="attribute") */
+    public $attributes;
+
+    public function addAttribute(string $name, string $value)
+    {
+        $this->attributes[$name] = new GH6531ArticleAttribute($name, $value, $this);
+    }
+}
+
+/**
+ * @Entity
+ */
+class GH6531ArticleAttribute
+{
+    /** @Id @ManyToOne(targetEntity=GH6531Article::class, inversedBy="attributes") */
+    public $article;
+
+    /** @Id @Column(type="string") */
+    public $attribute;
+
+    /** @Column(type="string") */
+    public $value;
+
+    public function __construct(string $name, string $value, GH6531Article $article)
+    {
+        $this->attribute = $name;
+        $this->value     = $value;
+        $this->article   = $article;
+    }
+}
+
+/**
+ * @Entity
+ */
+class GH6531Order
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+
+    /** @OneToMany(targetEntity=GH6531OrderItem::class, mappedBy="order", cascade={"ALL"}) */
+    public $items;
+
+    public function __construct()
+    {
+        $this->items = new ArrayCollection();
+    }
+
+    public function addItem(GH6531Product $product, int $amount) : void
+    {
+        $this->items->add(new GH6531OrderItem($this, $product, $amount));
+    }
+}
+
+/**
+ * @Entity
+ */
+class GH6531Product
+{
+    /** @Id @Column(type="integer") @GeneratedValue */
+    public $id;
+}
+
+/**
+ * @Entity
+ */
+class GH6531OrderItem
+{
+    /** @Id @ManyToOne(targetEntity=GH6531Order::class) */
+    public $order;
+
+    /** @Id @ManyToOne(targetEntity=GH6531Product::class) */
+    public $product;
+
+    /** @Column(type="integer") */
+    public $amount = 1;
+
+    public function __construct(GH6531Order $order, GH6531Product $product, int $amount = 1)
+    {
+        $this->order   = $order;
+        $this->product = $product;
+        $this->amount  = $amount;
+    }
+}