Execute aggregation framework on secondary node

Hello.

In C++ i tried execute aggregation framework like in example with readPreference :

conn->runCommand( "goods", BSON( "aggregate" << "events" << "pipeline" << pipeline<<"$readPreference"<<BSON("mode"<<"secondaryPreferred")), res);

Client connected to monogos with one single shard with four nodes in replicaset. Command always execute on primary node.

I dont understand, how execute framework on secondary node.
Is it possible?

Thank you.