fix(core): fix extracting PK out of reference wrapper

Closes #589

tests/issues/GH589.test.ts

@@ -0,0 +1,79 @@
+import { Collection, Entity, ManyToOne, MikroORM, OneToMany, PrimaryKey, PrimaryKeyType, Reference, IdentifiedReference } from '@mikro-orm/core';
+import { PostgreSqlDriver } from '@mikro-orm/postgresql';
+
+@Entity()
+export class User {
+
+  @PrimaryKey()
+  id!: number;
+
+  @OneToMany('Chat', 'owner')
+  ownedChats = new Collection<Chat>(this);
+
+}
+
+@Entity()
+export class Chat {
+
+  @ManyToOne(() => User, { primary: true, wrappedReference: true })
+  owner: IdentifiedReference<User>;
+
+  @ManyToOne(() => User, { primary: true, wrappedReference: true })
+  recipient: IdentifiedReference<User>;
+
+  [PrimaryKeyType]: [number, number];
+
+  constructor(owner: User, recipient: User) {
+    this.owner = Reference.create(owner);
+    this.recipient = Reference.create(recipient);
+  }
+
+}
+
+describe('GH issue 589', () => {
+
+  let orm: MikroORM<PostgreSqlDriver>;
+
+  beforeAll(async () => {
+    orm = await MikroORM.init({
+      entities: [User, Chat],
+      dbName: `mikro_orm_test_gh_589`,
+      type: 'postgresql',
+    });
+    await orm.getSchemaGenerator().ensureDatabase();
+    await orm.getSchemaGenerator().dropSchema();
+    await orm.getSchemaGenerator().createSchema();
+  });
+
+  afterAll(async () => {
+    await orm.close(true);
+  });
+
+  beforeEach(() => orm.em.clear());
+
+  test(`GH issue 589 (originally working)`, async () => {
+    const user1 = new User();
+    await orm.em.persistAndFlush(user1);
+    const user2 = new User();
+    await orm.em.persistAndFlush(user2);
+    const chat1 = new Chat(user1, user2);
+    orm.em.persist(chat1);
+    const chat2 = new Chat(user2, user1);
+    orm.em.persist(chat2);
+
+    await expect(orm.em.flush()).resolves.toBeUndefined();
+  });
+
+  test(`GH issue 589 (originally failing)`, async () => {
+    const user3 = new User();
+    await orm.em.persistAndFlush(user3);
+    const user4 = new User();
+    await orm.em.persistAndFlush(user4);
+    const chat3 = new Chat(user3, user4);
+    await orm.em.persistAndFlush(chat3);
+    const chat4 = new Chat(user4, user3);
+    orm.em.persist(chat4);
+
+    await expect(orm.em.flush()).resolves.toBeUndefined();
+  });
+});