Support pointer in aggregate query (#4493)

spec/ParseQuery.Aggregate.spec.js

@@ -96,6 +96,27 @@ describe('Parse.Query Aggregate testing', () => {
       }).catch(done.fail);
   });
 
+  it('group by pointer', (done) => {
+    const pointer1 = new TestObject();
+    const pointer2 = new TestObject();
+    const obj1 = new TestObject({ pointer: pointer1 });
+    const obj2 = new TestObject({ pointer: pointer2 });
+    const obj3 = new TestObject({ pointer: pointer1 });
+    const pipeline = [
+      { group: { objectId: '$pointer' } }
+    ];
+    Parse.Object.saveAll([pointer1, pointer2, obj1, obj2, obj3]).then(() => {
+      const query = new Parse.Query(TestObject);
+      return query.aggregate(pipeline);
+    }).then((results) => {
+      expect(results.length).toEqual(3);
+      expect(results.some(result => result.objectId === pointer1.id)).toEqual(true);
+      expect(results.some(result => result.objectId === pointer2.id)).toEqual(true);
+      expect(results.some(result => result.objectId === null)).toEqual(true);
+      done();
+    });
+  });
+
   it('group sum query', (done) => {
     const options = Object.assign({}, masterKeyOptions, {
       body: {

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -515,12 +515,26 @@ export class MongoStorageAdapter implements StorageAdapter {
   }
 
   aggregate(className: string, schema: any, pipeline: any, readPreference: ?string) {
+    let isPointerField = false;
+    pipeline = pipeline.map((stage) => {
+      if (stage.$group && stage.$group._id) {
+        const field = stage.$group._id.substring(1);
+        if (schema.fields[field] && schema.fields[field].type === 'Pointer') {
+          isPointerField = true;
+          stage.$group._id = `$_p_${field}`;
+        }
+      }
+      return stage;
+    });
     readPreference = this._parseReadPreference(readPreference);
     return this._adaptiveCollection(className)
       .then(collection => collection.aggregate(pipeline, { readPreference, maxTimeMS: this._maxTimeMS }))
       .then(results => {
         results.forEach(result => {
           if (result.hasOwnProperty('_id')) {
+            if (isPointerField && result._id) {
+              result._id = result._id.split('$')[1];
+            }
             result.objectId = result._id;
             delete result._id;
           }