Fix readpreference option parsing in MongoDB transport (#751)

* Fix readpreference option parsing in MongoDB transport

* Update changelog

Changelog

@@ -4,6 +4,17 @@
  Change history
 ================
 
+.. _version-4.0.3:
+
+4.0.3
+=====
+:release-date: TBA
+:release-by: Ask Solem
+
+- MongoDB: Fixed problem with using readPreference option at pymongo 3.x.
+
+    Contributed by **Mikhail Elovskikh**.
+
 .. _version-4.0.2:
 
 4.0.2

kombu/transport/mongodb.py

@@ -274,18 +274,21 @@ class Channel(virtual.Channel):
                                  if self.connect_timeout else None),
         }
         options.update(parsed['options'])
+        options = self._prepare_client_options(options)
 
         return hostname, dbname, options
 
     def _prepare_client_options(self, options):
         if pymongo.version_tuple >= (3,):
             options.pop('auto_start_request', None)
+            if isinstance(options.get('readpreference'), int):
+                modes = pymongo.read_preferences._MONGOS_MODES
+                options['readpreference'] = modes[options['readpreference']]
         return options
 
     def _open(self, scheme='mongodb://'):
-        hostname, dbname, options = self._parse_uri(scheme=scheme)
+        hostname, dbname, conf = self._parse_uri(scheme=scheme)
 
-        conf = self._prepare_client_options(options)
         conf['host'] = hostname
 
         env = _detect_environment()

t/unit/transport/test_mongodb.py

@@ -81,6 +81,12 @@ class test_mongodb_uri_parsing:
         assert hostname == 'mongodb://foo:bar@localhost/dbname'
         assert dbname == 'dbname'
 
+    def test_correct_readpreference(self):
+        url = 'mongodb://localhost/dbname?readpreference=nearest'
+        channel = _create_mock_connection(url).default_channel
+        hostname, dbname, options = channel._parse_uri()
+        assert options['readpreference'] == 'nearest'
+
 
 class BaseMongoDBChannelCase: