Set read_preference to primaryPreferred when topology single with replica set.

[ColaCheng]

We found the issue with querying the data when we use directConnection=true and the cluster is a replica set. We got this error by using the following steps.

iex> {:ok, top} = Mongo.start_link(url: "mongodb://localhost:27018/test?directConnection=true&readPreference=secondary&authSource=admin&maxPoolSize=1")
{:ok, #PID<0.230.0>}
iex> Mongo.aggregate(top, "test", [], [])
{:error,
 %Mongo.Error{
   message: "not primary and secondaryOk=false",
   code: 13435,
   host: nil,
   fail_command: false,
   error_labels: [],
   resumable: true,
   retryable_reads: true,
   retryable_writes: true,
   not_writable_primary_or_recovering: true,
   error_info: nil
 }}

Then I traced the Python MongoDB driver source code and found it set read_preference to primaryPreferred when topology single with replica set. Here is the reference.

After the changes, the query will work.

iex> {:ok, top} = Mongo.start_link(url: "mongodb://localhost:27018/test?directConnection=true&readPreference=secondary&authSource=admin&maxPoolSize=1")
{:ok, #PID<0.213.0>}
iex> Mongo.aggregate(top, "test", [], [])
%Mongo.Stream{
  topology_pid: #PID<0.213.0>,
  session: {:own, #PID<0.229.0>},
  cursor: 0,
  coll: "test.test",
  docs: [],
  cmd: [aggregate: "test", pipeline: [], cursor: %{}],
  opts: [session: #PID<0.229.0>, compression: true]
}

Please review it. Thank you!

[zookzook]

Looks good to me. Please call mix credo. Thank you!

[zookzook]

Did you execute mix credo? It should complain about the |> operator.

topology  |> mongos_servers() |> pick_server()

Looks nicer.

[ColaCheng]

Updated it. Thanks!

[zookzook]

topology  |> select_replica_set_server(read_preference.mode, read_preference) |> pick_server()

[ColaCheng]

Updated it. Thanks!