ReplSet: https://github.com/mongodb/node-mongodb-native/blob/1.4/lib/mongodb/connection/repl_set/repl_set.js#L161

vs

Server: https://github.com/mongodb/node-mongodb-native/blob/1.4/lib/mongodb/connection/server.js#L164

Which is used in: https://github.com/mongodb/node-mongodb-native/blob/1.4/lib/mongodb/db.js#L262

This causes readPreference variable to be missed out in the connect_options.

I don't know if this is intended or not, but after adding: this.options.readPreference = this._readPreference = readPreference;

I actually solved the problem and started to get reads from secondaries.

If this is not a bug, please provide an example on how should read preference be set in a sharded replicaset, because after trying countless of options I did not manage to make it work.