diff --git a/mongoengine/context_managers.py b/mongoengine/context_managers.py
index c6d0d40f..150f9657 100644
--- a/mongoengine/context_managers.py
+++ b/mongoengine/context_managers.py
@@ -1,7 +1,7 @@
 from mongoengine.common import _import_class
-from mongoengine.connection import DEFAULT_CONNECTION_NAME, get_db
-from pymongo.write_concern import WriteConcern
 from contextlib import contextmanager
+from pymongo.write_concern import WriteConcern
+from mongoengine.connection import DEFAULT_CONNECTION_NAME, get_db
 
 
 __all__ = ('switch_db', 'switch_collection', 'no_dereference',
diff --git a/mongoengine/document.py b/mongoengine/document.py
index 865bb063..2de0b1a3 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -854,7 +854,7 @@ class Document(BaseDocument):
         collection = cls._get_collection()
         # 746: when connection is via mongos, the read preference is not necessarily an indication that
         # this code runs on a secondary
-        if not collection.database.client.is_mongos and collection.read_preference > 1:
+        if not collection.is_mongos and collection.read_preference > 1:
             return
 
         # determine if an index which we are creating includes
diff --git a/tox.ini b/tox.ini
index 9bb0c5ec..2f2b1757 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,13 +1,11 @@
 [tox]
-envlist = {py27,py35,pypy,pypy3}-{mg27,mg28,mg35,mg3x}
+envlist = {py27,py35,pypy,pypy3}-{mg35,mg3x}
 
 [testenv]
 commands =
     python setup.py nosetests {posargs}
 deps =
     nose
-    mg27: PyMongo<2.8
-    mg28: PyMongo>=2.8,<2.9
     mg35: PyMongo==3.5
     mg3x: PyMongo>=3.0
 setenv =