gistfile1.js

var db = Knex({ /* connection properties */ });

function sessionToAccount(session) {
  return db('accounts').where('user_ID', session.user_ID).select();
}

function withdraw(account, amount) {
  if (account.balance < amount) {
    throw new Error("insufficient funds");
  }
  return db.raw('withdrawal(?,?)', account.ID, amount)
    .then(function(result) {
      return db('accounts').where('account', account.ID).select('balance')
        .then(function(account) {
          return({balance: account.balance, withdrawn: amount});
        });
    });
}

function getSession(session_id) {
  return db('sessions').where('session_id', session_id).select();
}

function handleWithdrawal(req, res) {
  getSession(req.param.session_id)
    .then(sessionToAccount)
    .then(function(account) {
      withdraw(account, req.param.amount);
    })
    .then(res.end)
    .catch(function(e) {
      res.send(500, "Withdrawal error: " + e);
    });
}