From 0c0822279a4c2187e28bff14c271bb6237f3acc1 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <valkar207@gmail.com>
Date: Mon, 10 Apr 2017 19:02:31 -0600
Subject: [PATCH 1/2] feat(aggregate): add mongoose-specific agg cursor

Fix #5145
---
 lib/aggregate.js       |  8 ++++++--
 test/aggregate.test.js | 13 +++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/lib/aggregate.js b/lib/aggregate.js
index 623fbaa8737..10c2210730e 100644
--- a/lib/aggregate.js
+++ b/lib/aggregate.js
@@ -2,10 +2,11 @@
  * Module dependencies
  */
 
-var util = require('util');
-var utils = require('./utils');
+var AggregationCursor = require('./cursor/AggregationCursor');
 var PromiseProvider = require('./promise_provider');
 var Query = require('./query');
+var util = require('util');
+var utils = require('./utils');
 var read = Query.prototype.read;
 
 /**
@@ -631,6 +632,9 @@ Aggregate.prototype.exec = function(callback) {
           callback && callback(null, cursor);
         });
       });
+    } else if (options.cursor.useMongooseAggCursor) {
+      delete options.cursor.useMongooseAggCursor;
+      return new AggregationCursor(this);
     }
     var cursor = this._model.collection.
       aggregate(this._pipeline, this.options || {});
diff --git a/test/aggregate.test.js b/test/aggregate.test.js
index 8bcadf83aa6..2350cadd017 100644
--- a/test/aggregate.test.js
+++ b/test/aggregate.test.js
@@ -898,6 +898,19 @@ describe('aggregate: ', function() {
     });
   });
 
+  it('cursor() with useMongooseAggCursor (gh-5145)', function(done) {
+    var db = start();
+
+    var MyModel = db.model('gh5145', { name: String });
+
+    var cursor = MyModel.
+      aggregate([{ $match: { name: 'test' } }]).
+      cursor({ useMongooseAggCursor: true }).
+      exec();
+    assert.ok(cursor instanceof require('stream').Readable);
+    done();
+  });
+
   it('cursor() eachAsync (gh-4300)', function(done) {
     var db = start();
 

From 894a5dd400643a4bf38042f5b1cf71c2b9156a6f Mon Sep 17 00:00:00 2001
From: Valeri Karpov <valkar207@gmail.com>
Date: Mon, 10 Apr 2017 19:05:20 -0600
Subject: [PATCH 2/2] chore: add missing files

---
 lib/cursor/AggregationCursor.js | 282 ++++++++++++++++++++++++++++++++
 1 file changed, 282 insertions(+)
 create mode 100644 lib/cursor/AggregationCursor.js

diff --git a/lib/cursor/AggregationCursor.js b/lib/cursor/AggregationCursor.js
new file mode 100644
index 00000000000..ce762b982b7
--- /dev/null
+++ b/lib/cursor/AggregationCursor.js
@@ -0,0 +1,282 @@
+/*!
+ * Module dependencies.
+ */
+
+var PromiseProvider = require('../promise_provider');
+var Readable = require('stream').Readable;
+var util = require('util');
+
+/**
+ * An AggregationCursor is a concurrency primitive for processing aggregation
+ * results one document at a time. It is analogous to QueryCursor.
+ *
+ * An AggregationCursor fulfills the [Node.js streams3 API](https://strongloop.com/strongblog/whats-new-io-js-beta-streams3/),
+ * in addition to several other mechanisms for loading documents from MongoDB
+ * one at a time.
+ *
+ * Unless you're an advanced user, do **not** instantiate this class directly.
+ * Use [`Aggregate#cursor()`](/docs/api.html#aggregate_Aggregate-cursor) instead.
+ *
+ * @param {Aggregate} agg
+ * @param {Object} options
+ * @inherits Readable
+ * @event `cursor`: Emitted when the cursor is created
+ * @event `error`: Emitted when an error occurred
+ * @event `data`: Emitted when the stream is flowing and the next doc is ready
+ * @event `end`: Emitted when the stream is exhausted
+ * @api public
+ */
+
+function AggregationCursor(agg) {
+  Readable.call(this, { objectMode: true });
+
+  this.cursor = null;
+  this.agg = agg;
+  this._transforms = [];
+  var _this = this;
+  var model = agg._model;
+  model.collection.aggregate(agg._pipeline, agg.options || {}, function(err, cursor) {
+    if (_this._error) {
+      cursor.close(function() {});
+      _this.listeners('error').length > 0 && _this.emit('error', _this._error);
+    }
+    if (err) {
+      return _this.emit('error', err);
+    }
+    _this.cursor = cursor;
+    _this.emit('cursor', cursor);
+  });
+}
+
+util.inherits(AggregationCursor, Readable);
+
+/*!
+ * Necessary to satisfy the Readable API
+ */
+
+AggregationCursor.prototype._read = function() {
+  var _this = this;
+  _next(this, function(error, doc) {
+    if (error) {
+      return _this.emit('error', error);
+    }
+    if (!doc) {
+      _this.push(null);
+      _this.cursor.close(function(error) {
+        if (error) {
+          return _this.emit('error', error);
+        }
+        setTimeout(function() {
+          _this.emit('close');
+        }, 0);
+      });
+      return;
+    }
+    _this.push(doc);
+  });
+};
+
+/**
+ * Registers a transform function which subsequently maps documents retrieved
+ * via the streams interface or `.next()`
+ *
+ * ####Example
+ *
+ *     // Map documents returned by `data` events
+ *     Thing.
+ *       find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *        doc.foo = "bar";
+ *        return doc;
+ *       })
+ *       on('data', function(doc) { console.log(doc.foo); });
+ *
+ *     // Or map documents returned by `.next()`
+ *     var cursor = Thing.find({ name: /^hello/ }).
+ *       cursor().
+ *       map(function (doc) {
+ *         doc.foo = "bar";
+ *         return doc;
+ *       });
+ *     cursor.next(function(error, doc) {
+ *       console.log(doc.foo);
+ *     });
+ *
+ * @param {Function} fn
+ * @return {QueryCursor}
+ * @api public
+ * @method map
+ */
+
+AggregationCursor.prototype.map = function(fn) {
+  this._transforms.push(fn);
+  return this;
+};
+
+/*!
+ * Marks this cursor as errored
+ */
+
+AggregationCursor.prototype._markError = function(error) {
+  this._error = error;
+  return this;
+};
+
+/**
+ * Marks this cursor as closed. Will stop streaming and subsequent calls to
+ * `next()` will error.
+ *
+ * @param {Function} callback
+ * @return {Promise}
+ * @api public
+ * @method close
+ * @emits close
+ * @see MongoDB driver cursor#close http://mongodb.github.io/node-mongodb-native/2.1/api/Cursor.html#close
+ */
+
+AggregationCursor.prototype.close = function(callback) {
+  var Promise = PromiseProvider.get();
+  var _this = this;
+  return new Promise.ES6(function(resolve, reject) {
+    _this.cursor.close(function(error) {
+      if (error) {
+        callback && callback(error);
+        reject(error);
+        return _this.listeners('error').length > 0 &&
+          _this.emit('error', error);
+      }
+      _this.emit('close');
+      resolve();
+      callback && callback();
+    });
+  });
+};
+
+/**
+ * Get the next document from this cursor. Will return `null` when there are
+ * no documents left.
+ *
+ * @param {Function} callback
+ * @return {Promise}
+ * @api public
+ * @method next
+ */
+
+AggregationCursor.prototype.next = function(callback) {
+  var Promise = PromiseProvider.get();
+  var _this = this;
+  return new Promise.ES6(function(resolve, reject) {
+    _next(_this, function(error, doc) {
+      if (error) {
+        callback && callback(error);
+        return reject(error);
+      }
+      callback && callback(null, doc);
+      resolve(doc);
+    });
+  });
+};
+
+/**
+ * Execute `fn` for every document in the cursor. If `fn` returns a promise,
+ * will wait for the promise to resolve before iterating on to the next one.
+ * Returns a promise that resolves when done.
+ *
+ * @param {Function} fn
+ * @param {Function} [callback] executed when all docs have been processed
+ * @return {Promise}
+ * @api public
+ * @method eachAsync
+ */
+
+AggregationCursor.prototype.eachAsync = function(fn, callback) {
+  var Promise = PromiseProvider.get();
+  var _this = this;
+
+  var handleNextResult = function(doc, callback) {
+    var promise = fn(doc);
+    if (promise && typeof promise.then === 'function') {
+      promise.then(
+        function() { callback(null); },
+        function(error) { callback(error); });
+    } else {
+      callback(null);
+    }
+  };
+
+  var iterate = function(callback) {
+    return _next(_this, function(error, doc) {
+      if (error) {
+        return callback(error);
+      }
+      if (!doc) {
+        return callback(null);
+      }
+      handleNextResult(doc, function(error) {
+        if (error) {
+          return callback(error);
+        }
+        // Make sure to clear the stack re: gh-4697
+        setTimeout(function() {
+          iterate(callback);
+        }, 0);
+      });
+    });
+  };
+
+  return new Promise.ES6(function(resolve, reject) {
+    iterate(function(error) {
+      if (error) {
+        callback && callback(error);
+        return reject(error);
+      }
+      callback && callback(null);
+      return resolve();
+    });
+  });
+};
+
+/*!
+ * Get the next doc from the underlying cursor and mongooseify it
+ * (populate, etc.)
+ */
+
+function _next(ctx, cb) {
+  var callback = cb;
+  if (ctx._transforms.length) {
+    callback = function(err, doc) {
+      if (err || doc === null) {
+        return cb(err, doc);
+      }
+      cb(err, ctx._transforms.reduce(function(doc, fn) {
+        return fn(doc);
+      }, doc));
+    };
+  }
+
+  if (ctx._error) {
+    return process.nextTick(function() {
+      callback(ctx._error);
+    });
+  }
+
+  if (ctx.cursor) {
+    return ctx.cursor.next(function(error, doc) {
+      if (error) {
+        return callback(error);
+      }
+      if (!doc) {
+        return callback(null, null);
+      }
+
+      callback(null, doc);
+    });
+  } else {
+    ctx.once('cursor', function() {
+      _next(ctx, cb);
+    });
+  }
+}
+
+module.exports = AggregationCursor;