feat(core): add @UseRequestContext() decorator

Closes #5

README.md

@@ -148,6 +148,34 @@ object.
 > still need CLI config with the full list of entities. On the other hand, we can
 > use globs there, as the CLI won't go thru webpack.
 
+## Request scoped handlers in queues
+
+As mentioned in the docs, we need a clean state for each request. That is handled
+automatically thanks to the `RequestContext` helper registered via middleware. 
+
+But middlewares are executed only for regular HTTP request handles, what if we need
+a request scoped method outside of that? One example of that is queue handlers or 
+scheduled tasks. 
+
+We can use the `@UseRequestContext()` decorator. It requires you to first inject the
+`MikroORM` instance to current context, it will be then used to create the context 
+for you. Under the hood, the decorator will register new request context for your 
+method and execute it inside the context. 
+
+```ts
+@Injectable()
+export class MyService {
+
+  constructor(private readonly orm: MikroORM) { }
+
+  @UseRequestContext()
+  async doSomething() {
+    // this will be executed in a separate context
+  }
+
+}
+```
+
 ## Using `AsyncLocalStorage` for request context
 
 By default, `domain` api use used in the `RequestContext` helper. Since `@mikro-orm/core@4.0.3`,

src/mikro-orm.common.ts

@@ -1,8 +1,29 @@
-import { AnyEntity, EntityName, MikroORM, Utils } from '@mikro-orm/core';
+import { AnyEntity, EntityName, MikroORM, RequestContext, Utils } from '@mikro-orm/core';
 import { Inject, Logger } from '@nestjs/common';
 
 export const MIKRO_ORM_MODULE_OPTIONS = Symbol('mikro-orm-module-options');
 export const REGISTERED_ENTITIES = new Set<EntityName<AnyEntity>>();
 export const logger = new Logger(MikroORM.name);
 export const getRepositoryToken = <T> (entity: EntityName<T>) => `${Utils.className(entity)}Repository`;
 export const InjectRepository = <T> (entity: EntityName<T>) => Inject(getRepositoryToken(entity));
+
+export function UseRequestContext() {
+  return function (target: any, propertyKey: string, descriptor: PropertyDescriptor) {
+    const originalMethod = descriptor.value;
+    descriptor.value = async function (...args: any[]) {
+      // eslint-disable-next-line @typescript-eslint/no-this-alias
+      const context: any = this;
+
+      if (!(context.orm instanceof MikroORM)) {
+        throw new Error('@UseRequestContext() decorator can only be applied to methods of classes that carry `orm: MikroORM`');
+      }
+
+      await RequestContext.createAsync(context.orm.em, async () => {
+        await originalMethod.apply(context, args);
+      });
+    };
+
+    return descriptor;
+  };
+}
+