How can I get a field count? about query-to-mongo HOT 1 CLOSED

var express = require('express')
var mongoskin = require('mongoskin')
var ObjectID = require('mongodb').ObjectID
var query2m = require('query-to-mongo')

// attach db to the router
var db = mongoskin.db('mongodb://localhost:27017/mydb', {safe: true})
var router = express.Router()
router.db = db
router.use(function (req, res, next) {
    req.db = router.db
    next()
})

// query objects in the collection
router.get('/my/query', function (req, res, next) {
    var query = query2m(req.query, { ignore: 'envelope' })
    var collection = req.db.collection('my')

    collection.count(query.criteria, function (e, count) {
        var links
        if (e) return next(e)
        res.append('X-Total-Count', count)
        var fullUrl = req.protocol + '://' + req.get('host') + req.originalUrl
        links = query.links(fullUrl, count)
        if (links) res.links(links)
        req.collection.aggregate([
            { $match: query.criteria },
            { $limit: query.options.limit },
            { $skip: query.options.skip },
            { $sort: query.options.sort },
            { $project: { item: 1, count: { $size:"$field" } }
        ]).toArray(function (e, results) {
            if (e) return next(e)
            res.locals.json = results
            next()
        })
    })
})

// attach the route to the app and serve
var app = express()
app.use('/api/formdata', router)
var server = app.listen(3000, function () {
    console.log('Listening on Port', server.address().port)
})