Convert mongo read_preference setting at a lower level.

Doing it at the modulestore level does not cover all usages of mongo,
for example when connecting to the contentstore.

common/lib/xmodule/xmodule/modulestore/django.py

@@ -24,7 +24,6 @@ import django.dispatch
 import django.utils
 from django.utils.translation import get_language, to_locale
 
-from pymongo import ReadPreference
 from xmodule.contentstore.django import contentstore
 from xmodule.modulestore.draft_and_published import BranchSettingMixin
 from xmodule.modulestore.mixed import MixedModuleStore
@@ -278,9 +277,6 @@ def create_modulestore_instance(
     else:
         xb_user_service = None
 
-    if 'read_preference' in doc_store_config:
-        doc_store_config['read_preference'] = getattr(ReadPreference, doc_store_config['read_preference'])
-
     xblock_field_data_wrappers = [load_function(path) for path in settings.XBLOCK_FIELD_DATA_WRAPPERS]
 
     def fetch_disabled_xblock_types():

common/lib/xmodule/xmodule/mongo_utils.py

@@ -4,6 +4,7 @@ Common MongoDB connection functions.
 import logging
 
 import pymongo
+from pymongo import ReadPreference
 from mongodb_proxy import MongoProxy
 
 logger = logging.getLogger(__name__)  # pylint: disable=invalid-name
@@ -33,6 +34,13 @@ def connect_to_mongodb(
         # No 'replicaSet' in kwargs - so no secondary reads.
         mongo_client_class = pymongo.MongoClient
 
+    # If read_preference is given as a name of a valid ReadPreference.<NAME> constant
+    # such as "SECONDARY_PREFERRED", convert it. Otherwise pass it through unchanged.
+    if 'read_preference' in kwargs:
+        read_preference = getattr(ReadPreference, kwargs['read_preference'], None)
+        if read_preference is not None:
+            kwargs['read_preference'] = read_preference
+
     mongo_conn = pymongo.database.Database(
         mongo_client_class(
             host=host,

common/lib/xmodule/xmodule/tests/test_mongo_utils.py

@@ -0,0 +1,28 @@
+"""Tests for methods defined in mongo_utils.py"""
+import os
+from unittest import TestCase
+from uuid import uuid4
+
+from pymongo import ReadPreference
+
+from django.conf import settings
+
+from xmodule.mongo_utils import connect_to_mongodb
+
+
+class MongoUtilsTests(TestCase):
+    """
+    Tests for methods exposed in mongo_utils
+    """
+    def test_connect_to_mongo_read_preference(self):
+        """
+        Test that read_preference parameter gets converted to a valid pymongo read preference.
+        """
+        host = 'edx.devstack.mongo' if 'BOK_CHOY_HOSTNAME' in os.environ else 'localhost'
+        db = 'test_read_preference_%s' % uuid4().hex
+        # Support for read_preference given in constant name form (ie. PRIMARY, SECONDARY_PREFERRED)
+        connection = connect_to_mongodb(db, host, read_preference='SECONDARY_PREFERRED')
+        self.assertEqual(connection.client.read_preference, ReadPreference.SECONDARY_PREFERRED)
+        # Support for read_preference given as mongos name.
+        connection = connect_to_mongodb(db, host, read_preference='secondaryPreferred')
+        self.assertEqual(connection.client.read_preference, ReadPreference.SECONDARY_PREFERRED)