fix(aggregation): ensure that the cursor key is always present

NODE-1117

lib/collection.js

@@ -21,8 +21,7 @@ var checkCollectionName = require('./utils').checkCollectionName
   , Cursor = require('./cursor')
   , unordered = require('./bulk/unordered')
   , ordered = require('./bulk/ordered')
-  , assign = require('./utils').assign
-  , mergeOptions = require('./utils').mergeOptions;
+  , assign = require('./utils').assign;
 
 /**
  * @fileOverview The **Collection** class is an internal class that embodies a MongoDB collection
@@ -2656,13 +2655,18 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
   options = getReadPreference(this, options, this.s.db, this);
 
   // If explain has been specified add it
-  if(options.explain) command.explain = options.explain;
+  if (options.explain) command.explain = options.explain;
 
   // Validate that cursor options is valid
   if(options.cursor != null && typeof options.cursor != 'object') {
     throw toError('cursor options must be an object');
   }
 
+  if (this.s.topology.capabilities().hasAggregationCursor) {
+    options.cursor = options.cursor || { batchSize : 1000 };
+    command.cursor = options.cursor;
+  }
+
   // promiseLibrary
   options.promiseLibrary = this.s.promiseLibrary;
 
@@ -2673,11 +2677,6 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
       throw new MongoError('cannot connect to server');
     }
 
-    if(this.s.topology.capabilities().hasAggregationCursor) {
-      options.cursor = options.cursor || { batchSize : 1000 };
-      command.cursor = options.cursor;
-    }
-
     // Allow disk usage command
     if(typeof options.allowDiskUse == 'boolean') command.allowDiskUse = options.allowDiskUse;
     if(typeof options.maxTimeMS == 'number') command.maxTimeMS = options.maxTimeMS;
@@ -2686,12 +2685,18 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
     return this.s.topology.cursor(this.s.namespace, command, options);
   }
 
-  // We do not allow cursor
-  if(options.cursor) {
-    return this.s.topology.cursor(this.s.namespace, command, options);
+  if (options.cursor) {
+    var cursor = this.s.topology.cursor(this.s.namespace, command, options);
+    return cursor.toArray(function(err, result) {
+      if (err) {
+        return handleCallback(callback, err);
+      }
+
+      handleCallback(callback, null, result);
+    });
   }
 
-  // Execute the command
+  // For legacy server versions, we execute the command and format the result
   this.s.db.command(command, options, function(err, result) {
     if(err) {
       handleCallback(callback, err);