Support pointer in distinct query (#4471)

* Support pointer in distinct query

* extract transform pointer string

spec/ParseQuery.Aggregate.spec.js

@@ -392,6 +392,23 @@ describe('Parse.Query Aggregate testing', () => {
       }).catch(done.fail);
   });
 
+  it('distinct 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 });
+    Parse.Object.saveAll([pointer1, pointer2, obj1, obj2, obj3]).then(() => {
+      const query = new Parse.Query(TestObject);
+      return query.distinct('pointer');
+    }).then((results) => {
+      expect(results.length).toEqual(2);
+      expect(results.some(result => result.objectId === pointer1.id)).toEqual(true);
+      expect(results.some(result => result.objectId === pointer2.id)).toEqual(true);
+      done();
+    });
+  });
+
   it('distinct class does not exist return empty', (done) => {
     const options = Object.assign({}, masterKeyOptions, {
       body: { distinct: 'unknown' }