Make queryset aggregates obey read_preference

mongoengine/queryset/base.py

@@ -1182,6 +1182,10 @@ class BaseQuerySet(object):
 
         pipeline = initial_pipeline + list(pipeline)
 
+        if IS_PYMONGO_3 and self._read_preference is not None:
+            return self._collection.with_options(read_preference=self._read_preference) \
+                       .aggregate(pipeline, cursor={}, **kwargs)
+
         return self._collection.aggregate(pipeline, cursor={}, **kwargs)
 
     # JS functionality

tests/queryset/queryset.py

@@ -4353,6 +4353,26 @@ class QuerySetTest(unittest.TestCase):
         self.assertEqual(bars._cursor._Cursor__read_preference,
                          ReadPreference.SECONDARY_PREFERRED)
 
+    @needs_mongodb_v26
+    def test_read_preference_aggregation_framework(self):
+        class Bar(Document):
+            txt = StringField()
+
+            meta = {
+                'indexes': ['txt']
+            }
+        # Aggregates with read_preference
+        bars = Bar.objects \
+                    .read_preference(ReadPreference.SECONDARY_PREFERRED) \
+                    .aggregate()
+        if IS_PYMONGO_3:
+            self.assertEqual(bars._CommandCursor__collection.read_preference,
+                             ReadPreference.SECONDARY_PREFERRED)
+        else:
+            self.assertNotEqual(bars._CommandCursor__collection.read_preference,
+                             ReadPreference.SECONDARY_PREFERRED)
+
+
     def test_json_simple(self):
 
         class Embedded(EmbeddedDocument):