[
    {
        username: "fabio",
        password: "xxx",
        roles: [
            {
                rolename: "administrator",
                isVip: true
            },
            {
                rolename: "manager",
                isVip: true
            }
        ]
    },
    {
        username: "marco",
        password: "yyy",
        roles: [
            {
                rolename: "manager",
                isVip: true
            }
        ]
    },
    {
        username: "pippo",
        password: "...",
        roles: [
            {
                rolename: "monkey",
                isVip: false
            }
        ]
    }
]

> db.users.distinct('roles.rolename')
[ "administrator", "manager", "monkey" ]