fix read_preference for PyMongo 3+

mongoengine/queryset/base.py

@@ -1444,6 +1444,14 @@ class BaseQuerySet(object):
     def _cursor(self):
         if self._cursor_obj is None:
 
+            # In PyMongo 3+, we define the read preference on a collection
+            # level, not a cursor level. Thus, we need to get a cloned
+            # collection object using `with_options` first.
+            if IS_PYMONGO_3 and self._read_preference is not None:
+                self._cursor_obj = self._collection\
+                    .with_options(read_preference=self._read_preference)\
+                    .find(self._query, **self._cursor_args)
+            else:
                 self._cursor_obj = self._collection.find(self._query,
                                                          **self._cursor_args)
             # Apply where clauses to cursor