cloudant - make couchDB calculate an average and keep it up to date when adding values -

is possible make couchdb (cloudant) calculate average rating value of each document ? if yes, how in cloudant ?

thanks.

{   "_id": "2016-02-06t13:16:30.515z",   "_rev": "3-7521b9e21fbb58d5393a76d08b12ab12",   "modelstrict": "taj model",   "ratings": {     "2016-02-06t13:36:04.671z": {       "useremail": "lourain@yaho.fr",       "rating": 1,       "sessionnumber": 0     },     "2016-02-06t13:46:04.671z": {       "useremail": "louin@yaoo.fr",       "rating": 3,       "sessionnumber": 0     },     "2016-02-06t13:53:04.671z": {       "useremail": "luin@yaoo.fr",       "rating": 3,       "sessionnumber": 0     },      "2016-02-06t13:47:04.671z": {       "useremail": "loin@yaoo.fr",       "rating": 5,       "sessionnumber": 0     }   },   "averagerating": ... } 

and how keep averagerating to date when rating added ?

an added rating looks this:

    "2016-02-06t17:57:04.671z": {       "useremail": "loin@yaoo.fr",       "rating": 2,       "sessionnumber": 10     } 

couchdb doesn't computed fields in documents. however, map/reduce perfect doing calculation incrementally, because couchdb efficient it's map/reduce calculations. (you can read more design decisions , impact on performance here)

my recommendation create separate documents each rating:

{   "modelstrict": "taj model",   "datetime": "2016-02-06t13:36:04.671z",   "useremail": "lourain@yaho.fr",   "rating": 1,   "sessionnumber": 0 }  {   "modelstrict": "taj model",   "datetime": "2016-02-06t13:46:04.671z",   "useremail": "louin@yaoo.fr",   "rating": 3,   "sessionnumber": 0 }  {   "modelstrict": "taj model",   "datetime": "2016-02-06t13:53:04.671z",   "useremail": "luin@yaoo.fr",   "rating": 3,   "sessionnumber": 0 }  {   "modelstrict": "taj model",   "datetime": "2016-02-06t13:47:04.671z",   "useremail": "loin@yaoo.fr",   "rating": 5,   "sessionnumber": 0 } 

then, can use map function this, group ratings modelstrict (which appears key)

function (doc) {   emit(doc.modelstrict, doc.rating); } 

then, can use reduce function calculating average: (i modified reduce function here

function(keys, values, rereduce) {     if (!rereduce) {         var length = values.length         return [sum(values) / length, length]     } else {         var length = sum(values.map(function(v){return v[1]}));         var avg = sum(values.map(function(v){             return v[0] * (v[1] / length)         }));         return [avg, length]     } } 

when call view, you'll [ 3, 4 ], average number of values used compute average. believe need return both both reduce , rereduce have enough context make calculation, 1 of weird couchdb-isms.

as use couchdb more , more, you'll find using more documents rather merging lot same document. makes writing views more flexible , more efficient on disk usage, of course experiment works best own application. hope helps!