read-preference only for read commands

Author: zookzook

.travis.yml

@@ -26,4 +26,4 @@ env:
     - MONGODB=3.2.20
     - MONGODB=3.4.16
     - MONGODB=3.6.6
-    - MONGODB=4.2
+    - MONGODB=ubuntu1804-v4.2-latest

test/mongo/topology_test.exs

@@ -14,7 +14,7 @@ defmodule Mongo.TopologyTest do
       rp = Mongo.ReadPreference.defaults(%{mode: mode})
       assert [%{"_id" => ^new_id, "topology_test" => 1}] =
                mongo_pid
-               |> Mongo.find("test", %{_id: new_id}, read_preference: rp)
+               |> Mongo.find("test", %{_id: new_id}, read_preference: rp, slave_ok: mode in [:secondary, :secondary_preferred])
                |> Enum.to_list
 
       assert {:ok, %Mongo.DeleteResult{deleted_count: 1}} = Mongo.delete_one(mongo_pid, "test", %{_id: new_id})