docs(transactions): add example of using transactions with aggregatio…

…n framework

Fix #6752

docs/transactions.jade

@@ -46,6 +46,17 @@ block content
     [require:transactions.*save]
     ```
 
+    ## With the Aggregation Framework
+
+    The `Model.aggregate()` function also supports transactions. Mongoose
+    aggregations have a [`session()` helper](/docs/api.html#aggregate_Aggregate-session)
+    that sets the [`session` option](/docs/api.html#aggregate_Aggregate-option).
+    Below is an example of executing an aggregation within a transaction.
+
+    ```javascript
+    [require:transactions.*aggregate]
+    ```
+
 block append layout
   script.
     _native.init("CK7DT53U", {

test/docs/transactions.test.js

@@ -97,4 +97,42 @@ describe('transactions', function() {
       }).
       then(doc => assert.ok(doc));
   });
+
+  it('aggregate', function() {
+    const Event = db.model('Event', new Schema({ createdAt: Date }), 'Event');
+
+    let session = null;
+    return db.createCollection('Event').
+      then(() => db.startSession()).
+      then(_session => {
+        session = _session;
+        session.startTransaction();
+        return Event.insertMany([
+          { createdAt: new Date('2018-06-01') },
+          { createdAt: new Date('2018-06-02') },
+          { createdAt: new Date('2017-06-01') },
+          { createdAt: new Date('2017-05-31') }
+        ], { session: session });
+      }).
+      then(() => Event.aggregate([
+        {
+          $group: {
+            _id: {
+              month: { $month: '$createdAt' },
+              year: { $year: '$createdAt' }
+            },
+            count: { $sum: 1 }
+          }
+        },
+        { $sort: { count: -1, '_id.year': -1, '_id.month': -1 } }
+      ]).session(session)).
+      then(res => {
+        assert.deepEqual(res, [
+          { _id: { month: 6, year: 2018 }, count: 2 },
+          { _id: { month: 6, year: 2017 }, count: 1 },
+          { _id: { month: 5, year: 2017 }, count: 1 }
+        ]);
+        session.commitTransaction();
+      });
+  });
 });