The implementation of subcommands is simple. For the add subcommand, we extract two arguments, USER_ID and ACTIVITY, and use them to create an Activity struct instance. We also get the current time with the Utc::now method and save it to a datetime field of Activity. Finally, we call the add_activity method to add the Activity instance to the MongoDB database:

match matches.subcommand() {
    (CMD_ADD, Some(matches)) => {
        let user_id = matches.value_of("USER_ID").unwrap().to_owned();
        let activity = matches.value_of("ACTIVITY").unwrap().to_owned();
        let activity = Activity {
            user_id,
            activity,
            datetime: Utc::now().to_string(),
        };
        add_activity(&conn, activity)?;
    }
    (CMD_LIST, _) => {
        let list = list_activities(&conn)?;
        for item in list {
            println!("User: {:20}    Activity: {:20}    DateTime: {:20}",
                     item.user_id, item.activity, item.datetime);
        }
    }
    _ => { matches.usage(); }
}

The list subcommand calls the list_activities function, and then iterates over all records to print them to a Terminal. The logging tool is finished – we can test it now.