add PROMO server-side validation route

[m-amaya]

No description provided.

[jrf0110]

This works nicely, but I think we can make it even better by consolidating the two database calls into one by adding a join:

var where = { promo_code, 'promos_applied.order_id': order_id };

var options = {
  joins:  [ { type: 'left'
            , target: 'promos_applied'
            , on: { promo_id: '$promos.id$' }
            }
          ]
};

db.promos.findOne( where, options, (err, result) => {
  if( !result ) {
    res.error( errors.promos.DOES_NOT_EXIST );
    return;
  }

  if( result.order_id ) {
    res.error( errors.promos.ALREADY_APPLIED );
    return;
  }

  res.sendStatus(204);
});

[jrf0110]

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