fix(core): add RequestContext.createAsync() for Koa

If the `next` handler needs to be awaited (like in Koa), one can now
use `RequestContext.createAsync()` method.

Closes #709

docs/docs/installation.md

@@ -159,6 +159,13 @@ app.use((req, res, next) => {
 });
 ```
 
+> If the `next` handler needs to be awaited (like in Koa), 
+> use `RequestContext.createAsync()` instead.
+>
+> ```typescript
+> app.use((ctx, next) => RequestContext.createAsync(orm.em, next));
+> ```
+
 More info about `RequestContext` is described [here](identity-map.md#request-context).
 
 ## Setting up the Commandline Tool

packages/core/src/utils/RequestContext.ts

@@ -14,13 +14,26 @@ export class RequestContext {
   /**
    * Creates new RequestContext instance and runs the code inside its domain.
    */
-  static create(em: EntityManager, next: (...args: any[]) => void) {
+  static create(em: EntityManager, next: (...args: any[]) => void): void {
     const context = new RequestContext(em.fork(true, true));
     const d = domain.create() as ORMDomain;
     d.__mikro_orm_context = context;
     d.run(next);
   }
 
+  /**
+   * Creates new RequestContext instance and runs the code inside its domain.
+   * Async variant, when the `next` handler needs to be awaited (like in Koa).
+   */
+  static async createAsync(em: EntityManager, next: (...args: any[]) => Promise<void>): Promise<void> {
+    const context = new RequestContext(em.fork(true, true));
+    const d = domain.create() as ORMDomain;
+    d.__mikro_orm_context = context;
+    await new Promise((resolve, reject) => {
+      d.run(() => next().then(resolve).catch(reject));
+    });
+  }
+
   /**
    * Returns current RequestContext (if available).
    */

tests/RequestContext.test.ts

@@ -22,6 +22,19 @@ describe('RequestContext', () => {
     expect(RequestContext.currentRequestContext()).toBeUndefined();
   });
 
+  test('create new context (async)', async () => {
+    expect(RequestContext.getEntityManager()).toBeUndefined();
+    await RequestContext.createAsync(orm.em, async () => {
+      const em = RequestContext.getEntityManager()!;
+      expect(em).not.toBe(orm.em);
+      // access UoW via property so we do not get the one from request context automatically
+      // @ts-ignore
+      expect(em.unitOfWork.getIdentityMap()).not.toBe(orm.em.unitOfWork.getIdentityMap());
+      expect(RequestContext.currentRequestContext()).not.toBeUndefined();
+    });
+    expect(RequestContext.currentRequestContext()).toBeUndefined();
+  });
+
   test('request context does not break population', async () => {
     const bible = new Book('Bible', new Author('God', 'hello@heaven.god'));
     const author = new Author('Jon Snow', 'snow@wall.st');