feat(AggregateRouter): support native mongodb syntax in aggregation pipelines (#7339)

spec/AggregateRouter.spec.js

@@ -1,6 +1,7 @@
 const AggregateRouter = require('../lib/Routers/AggregateRouter').AggregateRouter;
 
 describe('AggregateRouter', () => {
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline from Array', () => {
     const body = [
       {
@@ -12,6 +13,7 @@ describe('AggregateRouter', () => {
     expect(result).toEqual(expected);
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline from Object', () => {
     const body = {
       group: { objectId: {} },
@@ -21,6 +23,7 @@ describe('AggregateRouter', () => {
     expect(result).toEqual(expected);
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline from Pipeline Operator (Array)', () => {
     const body = {
       pipeline: [
@@ -34,6 +37,7 @@ describe('AggregateRouter', () => {
     expect(result).toEqual(expected);
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline from Pipeline Operator (Object)', () => {
     const body = {
       pipeline: {
@@ -45,6 +49,7 @@ describe('AggregateRouter', () => {
     expect(result).toEqual(expected);
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline fails multiple keys in Array stage ', () => {
     const body = [
       {
@@ -59,6 +64,7 @@ describe('AggregateRouter', () => {
     }
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get pipeline fails multiple keys in Pipeline Operator Array stage ', () => {
     const body = {
       pipeline: [
@@ -75,6 +81,7 @@ describe('AggregateRouter', () => {
     }
   });
 
+  // TODO: update pipeline syntax. See [#7339](https://bit.ly/3incnWx)
   it('get search pipeline from Pipeline Operator (Array)', () => {
     const body = {
       pipeline: {
@@ -85,4 +92,73 @@ describe('AggregateRouter', () => {
     const result = AggregateRouter.getPipeline(body);
     expect(result).toEqual(expected);
   });
+
+  it('support stage name starting with `$`', () => {
+    const body = {
+      $match: { someKey: 'whatever' },
+    };
+    const expected = [{ $match: { someKey: 'whatever' } }];
+    const result = AggregateRouter.getPipeline(body);
+    expect(result).toEqual(expected);
+  });
+
+  it('support nested stage names starting with `$`', () => {
+    const body = [
+      {
+        lookup: {
+          from: 'ACollection',
+          let: { id: '_id' },
+          as: 'results',
+          pipeline: [
+            {
+              $match: {
+                $expr: {
+                  $eq: ['$_id', '$$id'],
+                },
+              },
+            },
+          ],
+        },
+      },
+    ];
+    const expected = [
+      {
+        $lookup: {
+          from: 'ACollection',
+          let: { id: '_id' },
+          as: 'results',
+          pipeline: [
+            {
+              $match: {
+                $expr: {
+                  $eq: ['$_id', '$$id'],
+                },
+              },
+            },
+          ],
+        },
+      },
+    ];
+    const result = AggregateRouter.getPipeline(body);
+    expect(result).toEqual(expected);
+  });
+
+  it('support the use of `_id` in stages', () => {
+    const body = [
+      { match: { _id: 'randomId' } },
+      { sort: { _id: -1 } },
+      { addFields: { _id: 1 } },
+      { group: { _id: {} } },
+      { project: { _id: 0 } },
+    ];
+    const expected = [
+      { $match: { _id: 'randomId' } },
+      { $sort: { _id: -1 } },
+      { $addFields: { _id: 1 } },
+      { $group: { _id: {} } },
+      { $project: { _id: 0 } },
+    ];
+    const result = AggregateRouter.getPipeline(body);
+    expect(result).toEqual(expected);
+  });
 });