Support Distinct for special fields (#5144)

* Support Distinct for special fields

* update changelog

CHANGELOG.md

@@ -5,7 +5,7 @@
 
 #### Improvements:
 * Fixes issue that would prevent users with large number of roles to resolve all of them [@Moumouls]() (#5131, #5132)
-
+* Fixes distinct query on special fields ([#5144](https://github.com/parse-community/parse-server/pull/5144))
 
 ### 3.1.0
 [Full Changelog](https://github.com/parse-community/parse-server/compare/3.0.0...3.1.0)

spec/ParseQuery.Aggregate.spec.js

@@ -1098,6 +1098,36 @@ describe('Parse.Query Aggregate testing', () => {
       .catch(done.fail);
   });
 
+  it('distinct objectId', async () => {
+    const query = new Parse.Query(TestObject);
+    const results = await query.distinct('objectId');
+    expect(results.length).toBe(4);
+  });
+
+  it('distinct createdAt', async () => {
+    const object1 = new TestObject({ createdAt_test: true });
+    await object1.save();
+    const object2 = new TestObject({ createdAt_test: true });
+    await object2.save();
+    const query = new Parse.Query(TestObject);
+    query.equalTo('createdAt_test', true);
+    const results = await query.distinct('createdAt');
+    expect(results.length).toBe(2);
+  });
+
+  it('distinct updatedAt', async () => {
+    const object1 = new TestObject({ updatedAt_test: true });
+    await object1.save();
+    const object2 = new TestObject();
+    await object2.save();
+    object2.set('updatedAt_test', true);
+    await object2.save();
+    const query = new Parse.Query(TestObject);
+    query.equalTo('updatedAt_test', true);
+    const results = await query.distinct('updatedAt');
+    expect(results.length).toBe(2);
+  });
+
   it('distinct null field', done => {
     const options = Object.assign({}, masterKeyOptions, {
       body: { distinct: 'distinctField' },

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -710,19 +710,20 @@ export class MongoStorageAdapter implements StorageAdapter {
     schema = convertParseSchemaToMongoSchema(schema);
     const isPointerField =
       schema.fields[fieldName] && schema.fields[fieldName].type === 'Pointer';
-    if (isPointerField) {
+    const transformField = transformKey(className, fieldName, schema);
-      fieldName = `_p_${fieldName}`;
+
-    }
     return this._adaptiveCollection(className)
       .then(collection =>
-        collection.distinct(fieldName, transformWhere(className, query, schema))
+        collection.distinct(
+          transformField,
+          transformWhere(className, query, schema)
+        )
       )
       .then(objects => {
         objects = objects.filter(obj => obj != null);
         return objects.map(object => {
           if (isPointerField) {
-            const field = fieldName.substring(3);
+            return transformPointerString(schema, fieldName, object);
-            return transformPointerString(schema, field, object);
           }
           return mongoObjectToParseObject(className, object, schema);
         });