Mongoose set local variable from query

I have a function which basically queries the MongoDB for a credential (simplified):

var query = Credential.findOne({token: token});

return query.exec(function (err, credential) {
    if (err) return null;
    return credential;
});

I need this credential for another query to get associated roles:

router.get('/api/roles', function (req, res) {
   var token = req.headers.authorization || '';
   var cred;

   if (token === '') {
       return res.sendStatus(401);
   }
   getCredential(token).then(function (credential) {
      cred = credential;
   });

   getRoles(cred).then(function(roles) {
      return res.json(roles);
   });
});

Unfortunately, I can't seem to set the local variable cred from within the resolved promise and thus it is undefined. Any idea how to do this?