mongo shell connect to secondary

I am trying to run periodically a somehow heavy script in a mongo replica set.

As we are not using secondaries for read operations, I'd like to execute it against a secondary.

So I have a js script that can be executed in mongo shell like this :

mongo <replica_name>/host1:port1,host1:port2/<dbname> <my_script.js>

Inside my script I have :

db.getMongo().setReadPref('secondary');

Is this enough to ensure I will be executing against a secondary?

For instance if I execute

mongo <replica_name>/host1:port1,host1:port2/<dbname> 

I get a prompt that says I am in the primary

rs_name:PRIMARY> db.getMongo().setReadPref('secondary');
rs_name:PRIMARY> db.getMongo()
connection to rs_name/localhost:27117,localhost:27127

So I don't know where my read operations are going to be executed.

Any ideas??