feat(core): allow forking EM without clearing identity map

With this one can easily run parallel branches that will have its own identity map (keeping the old entities present so flushing will work as expected).

lib/EntityManager.ts

@@ -288,8 +288,14 @@ export class EntityManager {
     return ret;
   }
 
-  fork(): EntityManager {
-    return new EntityManager(this.config, this.driver);
+  fork(clear = true): EntityManager {
+    const em = new EntityManager(this.config, this.driver);
+
+    if (!clear) {
+      Object.values(this.getUnitOfWork().getIdentityMap()).forEach(entity => em.merge(entity));
+    }
+
+    return em;
   }
 
   getUnitOfWork(): UnitOfWork {

tests/EntityManager.mysql.test.ts

@@ -923,6 +923,22 @@ describe('EntityManagerMySql', () => {
     expect(b1.toJSON()).toMatchObject({ fooBar: b1.id });
   });
 
+  test('persisting entities in parallel inside forked EM with copied IM', async () => {
+    const author = new Author2('name', 'email');
+    await orm.em.persistAndFlush(author); // we need to flush here so the entity gets inside IM
+
+    // fork EM without clearing the IM (once for each process), so author entity will be there
+    await Promise.all([
+      orm.em.fork(false).persistAndFlush(new Book2('b1', author)),
+      orm.em.fork(false).persistAndFlush(new Book2('b2', author)),
+      orm.em.fork(false).persistAndFlush(new Book2('b3', author)),
+    ]);
+
+    orm.em.clear();
+    const a1 = (await orm.em.findOne(Author2, author.id, { populate: ['books'] }))!;
+    expect(a1.books.count()).toBe(3);
+  });
+
   test('EM supports smart search conditions', async () => {
     const author = new Author2('name', 'email');
     const b1 = new Book2('b1', author);