db.createUser(
  {
    user: "admin",
    pwd: "MioTech4ever!",
    roles: [ { role: "userAdminAnyDatabase", db: "admin" } ]
  }
)

db.updateUser( "admin",
  {
    roles: [ { role: "userAdminAnyDatabase", db: "admin" },
{ role: "readWrite", db: "BookstoreDb" },
{ role: "readWrite", db: "MyCoreDb" } ]
  }
)