Assume followings:
/mykeys/.ssh/prodserver.pem - is a certificate file
umid - is a user name
111.111.111.111 - is a remote host, that mongodb runs
2222 - is a remote port to connect via ssh
localhost:27017 - local host and port on remote machine
27018 - local port
ssh -i /mykeys/.ssh/prodserver.pem [email protected] -p 2222 -Nf -L 27018:localhost:27017
Connect to the mongodb via opened tunnel, in this case mydb is database name
mongo --port 27018 mydb