fix(cursor): transforms should only be applied once to documents

A regression in merging the core cursor into the native codebase
resultied in a document transform (set with the `.map` helper) to
be applied to incoming documents twice.

NODE-2503

lib/core/cursor.js

@@ -359,15 +359,6 @@ class CoreCursor extends Readable {
         return this.push(this.cursorState.streamOptions.transform(result));
       }
 
-      // If we provided a map function
-      if (
-        this.cursorState.transforms &&
-        typeof this.cursorState.transforms.doc === 'function' &&
-        result != null
-      ) {
-        return this.push(this.cursorState.transforms.doc(result));
-      }
-
       // Return the result
       this.push(result);
 

lib/cursor.js

@@ -819,6 +819,7 @@ class Cursor extends CoreCursor {
         driver: true
       });
     }
+
     return maybePromise(this, callback, cb => {
       const cursor = this;
       const items = [];
@@ -846,12 +847,6 @@ class Cursor extends CoreCursor {
           // Get all buffered objects
           if (cursor.bufferedCount() > 0) {
             let docs = cursor.readBufferedDocuments(cursor.bufferedCount());
-
-            // Transform the doc if transform method added
-            if (cursor.s.transforms && typeof cursor.s.transforms.doc === 'function') {
-              docs = docs.map(cursor.s.transforms.doc);
-            }
-
             Array.prototype.push.apply(items, docs);
           }
 

test/functional/cursor.test.js

@@ -4609,4 +4609,58 @@ describe('Cursor', function() {
       });
     });
   });
+
+  describe('transforms', function() {
+    it('should correctly apply map transform to cursor as readable stream', function(done) {
+      const configuration = this.configuration;
+      const client = configuration.newClient();
+      client.connect(err => {
+        expect(err).to.not.exist;
+        this.defer(() => client.close());
+
+        const docs = 'Aaden Aaron Adrian Aditya Bob Joe'.split(' ').map(x => ({ name: x }));
+        const coll = client.db(configuration.db).collection('cursor_stream_mapping');
+        coll.insertMany(docs, err => {
+          expect(err).to.not.exist;
+
+          const bag = [];
+          const stream = coll
+            .find()
+            .project({ _id: 0, name: 1 })
+            .map(doc => ({ mapped: doc }))
+            .on('data', doc => bag.push(doc));
+
+          stream.on('error', done).on('end', () => {
+            expect(bag.map(x => x.mapped)).to.eql(docs.map(x => ({ name: x.name })));
+            done();
+          });
+        });
+      });
+    });
+
+    it('should correctly apply map transform when converting cursor to array', function(done) {
+      const configuration = this.configuration;
+      const client = configuration.newClient();
+      client.connect(err => {
+        expect(err).to.not.exist;
+        this.defer(() => client.close());
+
+        const docs = 'Aaden Aaron Adrian Aditya Bob Joe'.split(' ').map(x => ({ name: x }));
+        const coll = client.db(configuration.db).collection('cursor_toArray_mapping');
+        coll.insertMany(docs, err => {
+          expect(err).to.not.exist;
+
+          coll
+            .find()
+            .project({ _id: 0, name: 1 })
+            .map(doc => ({ mapped: doc }))
+            .toArray((err, mappedDocs) => {
+              expect(err).to.not.exist;
+              expect(mappedDocs.map(x => x.mapped)).to.eql(docs.map(x => ({ name: x.name })));
+              done();
+            });
+        });
+      });
+    });
+  });
 });