feat(AggregateRouter): support native mongodb syntax in aggregation pipelines

CHANGELOG.md

@@ -102,6 +102,7 @@ ___
 - EXPERIMENTAL: Added custom routes to easily customize flows for password reset, email verification or build entirely new flows (Manuel Trezza) [#7231](https://github.com/parse-community/parse-server/issues/7231)
 - Added Deprecation Policy to govern the introduction of braking changes in a phased pattern that is more predictable for developers (Manuel Trezza) [#7199](https://github.com/parse-community/parse-server/pull/7199)
 ### Other Changes
+- Support aggregation stage names starting with '$' just like in MongoDB (Raschid JF Rafeally) [#7339](https://github.com/parse-community/parse-server/pull/7339)
 - Fix error when a not yet inserted job is updated (Antonio Davi Macedo Coelho de Castro) [#7196](https://github.com/parse-community/parse-server/pull/7196)
 - request.context for afterFind triggers (dblythy) [#7078](https://github.com/parse-community/parse-server/pull/7078)
 - Winston Logger interpolating stdout to console (dplewis) [#7114](https://github.com/parse-community/parse-server/pull/7114)

spec/AggregateRouter.spec.js

@@ -85,4 +85,22 @@ describe('AggregateRouter', () => {
     const result = AggregateRouter.getPipeline(body);
     expect(result).toEqual(expected);
   });
+
+  it("support stage name starting with '$'", () => {
+    const body = {
+      $match: { someKey: 'whatever' },
+    };
+    const expected = [{ $match: { someKey: 'whatever' } }];
+    const result = AggregateRouter.getPipeline(body);
+    expect(result).toEqual(expected);
+  });
+
+  it("support stage name not starting with '$'", () => {
+    const body = {
+      match: { someKey: 'whatever' },
+    };
+    const expected = [{ $match: { someKey: 'whatever' } }];
+    const result = AggregateRouter.getPipeline(body);
+    expect(result).toEqual(expected);
+  });
 });

src/Routers/AggregateRouter.js

@@ -106,7 +106,8 @@ export class AggregateRouter extends ClassesRouter {
       stage[stageName]._id = stage[stageName].objectId;
       delete stage[stageName].objectId;
     }
-    return { [`$${stageName}`]: stage[stageName] };
+    const key = stageName[0] === '$' ? stageName : `$${stageName}`;
+    return { [key]: stage[stageName] };
   }
 
   mountRoutes() {