feat(cursor): new cursor.transformStream method

Created a new method, cursor.transformStream, which
returns a readable stream with the transformation
applied. Does not mutate the cursor.

Fixes NODE-1208

lib/cursor.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const Transform = require('stream').Transform;
+const PassThrough = require('stream').PassThrough;
 const inherits = require('util').inherits;
 const f = require('util').format;
 const deprecate = require('util').deprecate;
@@ -1124,15 +1126,40 @@ Cursor.prototype.destroy = function(err) {
 /**
  * Return a modified Readable stream including a possible transform method.
  * @method
- * @param {object} [options=null] Optional settings.
- * @param {function} [options.transform=null] A transformation method applied to each document emitted by the stream.
+ * @param {object} [options] Optional settings.
+ * @param {function} [options.transform] A transformation method applied to each document emitted by the stream.
  * @return {Cursor}
+ * TODO: replace this method with transformStream in next major release
  */
 Cursor.prototype.stream = function(options) {
   this.s.streamOptions = options || {};
   return this;
 };
 
+/**
+ * Return a modified Readable stream that applies a given transform function, if supplied. If none supplied,
+ * returns a stream of unmodified docs.
+ * @method
+ * @param {object} [options] Optional settings.
+ * @param {function} [options.transform] A transformation method applied to each document emitted by the stream.
+ * @return {stream}
+ */
+Cursor.prototype.transformStream = function(options) {
+  const streamOptions = options || {};
+  if (typeof streamOptions.transform === 'function') {
+    const stream = new Transform({
+      objectMode: true,
+      transform: function(chunk, encoding, callback) {
+        this.push(streamOptions.transform(chunk));
+        callback();
+      }
+    });
+
+    return this.pipe(stream);
+  }
+  return this.pipe(new PassThrough({ objectMode: true }));
+};
+
 /**
  * Execute the explain for the cursor
  * @method

test/functional/cursor_tests.js

@@ -4423,4 +4423,82 @@ describe('Cursor', function() {
       });
     });
   });
+
+  function testTransformStream(config, done) {
+    const client = config.client;
+    const configuration = config.configuration;
+    const collectionName = config.collectionName;
+    const transformFunc = config.transformFunc;
+    const expectedSet = config.expectedSet;
+
+    client.connect(function(err, client) {
+      const db = client.db(configuration.db);
+      let collection, cursor;
+      const docs = [
+        { _id: 0, a: { b: 1, c: 0 } },
+        { _id: 1, a: { b: 1, c: 0 } },
+        { _id: 2, a: { b: 1, c: 0 } }
+      ];
+      const resultSet = new Set();
+      const transformParam = transformFunc != null ? { transform: transformFunc } : null;
+      const close = e => cursor.close(() => client.close(() => done(e)));
+
+      Promise.resolve()
+        .then(() => db.createCollection(collectionName))
+        .then(() => (collection = db.collection(collectionName)))
+        .then(() => collection.insertMany(docs))
+        .then(() => collection.find())
+        .then(_cursor => (cursor = _cursor))
+        .then(() => cursor.transformStream(transformParam))
+        .then(stream => {
+          stream.on('data', function(doc) {
+            resultSet.add(doc);
+          });
+
+          stream.once('end', function() {
+            expect(resultSet).to.deep.equal(expectedSet);
+            close();
+          });
+
+          stream.once('error', function(e) {
+            close(e);
+          });
+        })
+        .catch(e => close(e));
+    });
+  }
+
+  it('transformStream should apply the supplied transformation function to each document in the stream', function(done) {
+    const configuration = this.configuration;
+    const client = configuration.newClient({ w: 1 }, { poolSize: 1, auto_reconnect: false });
+    const expectedDocs = [{ _id: 0, b: 1, c: 0 }, { _id: 1, b: 1, c: 0 }, { _id: 2, b: 1, c: 0 }];
+    const config = {
+      client: client,
+      configuration: configuration,
+      collectionName: 'transformStream-test-transform',
+      transformFunc: doc => ({ _id: doc._id, b: doc.a.b, c: doc.a.c }),
+      expectedSet: new Set(expectedDocs)
+    };
+
+    testTransformStream(config, done);
+  });
+
+  it('transformStream should return a stream of unmodified docs if no transform function applied', function(done) {
+    const configuration = this.configuration;
+    const client = configuration.newClient({ w: 1 }, { poolSize: 1, auto_reconnect: false });
+    const expectedDocs = [
+      { _id: 0, a: { b: 1, c: 0 } },
+      { _id: 1, a: { b: 1, c: 0 } },
+      { _id: 2, a: { b: 1, c: 0 } }
+    ];
+    const config = {
+      client: client,
+      configuration: configuration,
+      collectionName: 'transformStream-test-notransform',
+      transformFunc: null,
+      expectedSet: new Set(expectedDocs)
+    };
+
+    testTransformStream(config, done);
+  });
 });