Allow select (keys) to be altered in triggers (#3146)

Inspect the keys when a query is returned from a trigger and respect the new
value.

spec/CloudCode.spec.js

@@ -1434,5 +1434,40 @@ describe('afterFind hooks', () => {
     });
   });
 
-});
+  it('should alter select', (done) => {
+    Parse.Cloud.beforeFind('MyObject', (req) => {
+      req.query.select('white');
+      return req.query;
+    });
 
+    const obj0 = new Parse.Object('MyObject')
+      .set('white', true)
+      .set('black', true);
+    obj0.save()
+      .then(() => {
+        new Parse.Query('MyObject')
+          .first()
+          .then(result => {
+            expect(result.get('white')).toBe(true);
+            expect(result.get('black')).toBe(undefined);
+            done();
+          });
+      });
+  });
+
+  it('should not alter select', (done) => {
+    const obj0 = new Parse.Object('MyObject')
+      .set('white', true)
+      .set('black', true);
+    obj0.save()
+      .then(() => {
+        new Parse.Query('MyObject')
+          .first()
+          .then(result => {
+            expect(result.get('white')).toBe(true);
+            expect(result.get('black')).toBe(true);
+            done();
+          });
+      });
+  });
+});

src/triggers.js

@@ -333,6 +333,10 @@ export function maybeRunQueryTrigger(triggerType, className, restWhere, restOpti
       restOptions = restOptions || {};
       restOptions.include = jsonQuery.include;
     }
+    if (jsonQuery.keys) {
+      restOptions = restOptions || {};
+      restOptions.keys = jsonQuery.keys;
+    }
     return {
       restWhere,
       restOptions