javascript - Node and Mongoose create/use field that increases +1 for each new added -

i'm going try , keep simple possible. i've spent day testing modules this, , @ point have gotten confused figured best option.

i have schema:

var appschema = new mongoose.schema({     appid: { type: number },     appname: { type: string },     appdesc: { type: string } }); app.db.model('userapp', appschema); 

using express , processing post, want create "a new app"

app.post('/app/new', function (req, res) {     var newapp = app.db.models.userapp({         appid: functiontoincreasebyone(),         appname: req.param('appname'),         appdesc: req.param('appdesc')     });     newapp.save(); }); 

i removed of bloat in there, redirects, etc, keep simple. have appid set @ 1 first first app added, , increase +1 each new app added. seems simple i've gone through many complex examples head hurts now.

i thought using .find , orderby find app highest appid, use , add +1 it, i'm sure there has another, easier way. don't want appid change when there update, time want set on first post route create new app.

if please me out here or @ least point me in right direction, appreciate it.

i can see several ways so.

first, may store last_id in global variable:

var last_id = 0;  app.post('/app/new', function (req, res) {   var newapp = app.db.models.userapp({     appid: ++last_id,     appname: req.param('appname'),     appdesc: req.param('appdesc')   });   newapp.save(); }); 

you may add proper initialization of last_id, it'll "catch up" database when application starts.

the disadvantage of approach can't use if have several node.js processes.

second, may query database each time want create new document:

app.post('/app/new', function (req, res, next) {   app.db.models.userapp.find()     .sort({_id: -1})     .limit(1)     .select('_id')     .exec(function (err, docs) {       if (err) return next(err);       var newapp = app.db.models.userapp({         appid: docs[0]._id,         appname: req.param('appname'),         appdesc: req.param('appdesc')       });       newapp.save();     }) }); 

but there no way guarantee next find operation occur after previous save operation. so, there chance app try 2 save 2 different docs 1 _id. it's why should avoid solution if possible.

third, may use other storage (e.g. redis) atomically increment last_id every time you're reading it:

var redisclient = require('redis').createclient()  app.post('/app/new', function (req, res, next) {   redisclient.incr('last_id', function (err, next_id) {     if (err) return next(err);     var newapp = app.db.models.userapp({       appid: next_id,       appname: req.param('appname'),       appdesc: req.param('appdesc')     });     newapp.save();   }) }); 

this solution best one, require additional database installed.